More optimizations for reagent code (#80529)

## About The Pull Request

1. Removed offsetting reagent volumes by the quantization/rounding
constants before comparing values. Not required since the final total
volume is rounded inside the reagent holder anyway.
2. Removed excessive calls to the hottest proc in reagent code i.e.
`update_total()` per reaction step. Speeds up reactions even more.
3. Removed rounding of volumes (like in liver, pyrotechnics, overheated
reactions) where the volume is already rounded inside the holder.
4. Avoid calling `handle_reactions()` if the holder is already reacting
when adding new reagents.

## Changelog
🆑
code: removed excess calls to `update_total()` making reaction code
slightly faster.
code: removed excessive use of chemical constants (quantisation &
rounding constants) in places where they were not needed i.e. plumbing
buffer, reaction chamber, pyrotechnics & the liver.
/🆑

code/modules/plumbing/plumbers/plumbing_buffer.dm

@@ -33,11 +33,11 @@
 	SIGNAL_HANDLER
 	if(!buffer_net)
 		return
-	if(reagents.total_volume + CHEMICAL_QUANTISATION_LEVEL >= activation_volume && mode == UNREADY)
+	if(reagents.total_volume >= activation_volume && mode == UNREADY)
 		mode = IDLE
 		buffer_net.check_active()
 
-	else if(reagents.total_volume + CHEMICAL_QUANTISATION_LEVEL < activation_volume && mode != UNREADY)
+	else if(reagents.total_volume < activation_volume && mode != UNREADY)
 		mode = UNREADY
 		buffer_net.check_active()
 

code/modules/plumbing/plumbers/reaction_chamber.dm

@@ -43,7 +43,7 @@
 /obj/machinery/plumbing/reaction_chamber/proc/on_reagent_change(datum/reagents/holder, ...)
 	SIGNAL_HANDLER
 
-	if(holder.total_volume <= CHEMICAL_VOLUME_ROUNDING && emptying) //we were emptying, but now we aren't
+	if(!holder.total_volume && emptying) //we were emptying, but now we aren't
 		emptying = FALSE
 		holder.flags |= NO_REACT
 	return NONE

code/modules/reagents/chemistry/holder/holder.dm

@@ -129,7 +129,6 @@
 		amount = adjusted_vol
 		has_split = TRUE
 
-	update_total()
 	var/cached_total = total_volume
 	if(cached_total + amount > maximum_volume)
 		amount = maximum_volume - cached_total //Doesnt fit in. Make it disappear. shouldn't happen. Will happen.
@@ -195,7 +194,7 @@
 			set_temperature(reagtemp)
 
 	SEND_SIGNAL(src, COMSIG_REAGENTS_NEW_REAGENT, new_reagent, amount, reagtemp, data, no_react)
-	if(!no_react)
+	if(!no_react && !is_reacting)
 		handle_reactions()
 	return amount
 

code/modules/reagents/chemistry/holder/reactions.dm

@@ -190,7 +190,6 @@
 	if(!LAZYLEN(reaction_list))
 		finish_reacting()
 	else
-		update_total()
 		handle_reactions()
 
 /*

code/modules/reagents/chemistry/recipes.dm

@@ -182,7 +182,8 @@
 		var/datum/reagent/reagent = holder.has_reagent(id)
 		if(!reagent)
 			return
-		reagent.volume = round((reagent.volume * 0.98), CHEMICAL_QUANTISATION_LEVEL) //Slowly lower yield per tick
+		reagent.volume *= 0.98 //Slowly lower yield per tick
+	holder.update_total()
 
 /**
  * Occurs when a reation is too impure (i.e. it's below purity_min)

code/modules/reagents/chemistry/recipes/pyrotechnics.dm

@@ -15,7 +15,7 @@
 
 /datum/chemical_reaction/reagent_explosion/nitroglycerin/on_reaction(datum/reagents/holder, datum/equilibrium/reaction, created_volume)
 
-	if(holder.has_reagent(/datum/reagent/exotic_stabilizer,round(created_volume / 25, CHEMICAL_QUANTISATION_LEVEL)))
+	if(holder.has_reagent(/datum/reagent/exotic_stabilizer, created_volume / 25))
 		return
 	holder.remove_reagent(/datum/reagent/nitroglycerin, created_volume * 2)
 	..()
@@ -78,10 +78,10 @@
 	strengthdiv = 3
 
 /datum/chemical_reaction/reagent_explosion/tatp/update_info()
-	required_temp = 450 + rand(-49,49)  //this gets loaded only on round start
+	required_temp = 450 + rand(-49, 49)  //this gets loaded only on round start
 
 /datum/chemical_reaction/reagent_explosion/tatp/on_reaction(datum/reagents/holder, datum/equilibrium/reaction, created_volume)
-	if(holder.has_reagent(/datum/reagent/exotic_stabilizer,round(created_volume / 50, CHEMICAL_QUANTISATION_LEVEL))) // we like exotic stabilizer
+	if(holder.has_reagent(/datum/reagent/exotic_stabilizer, created_volume / 50)) // we like exotic stabilizer
 		return
 	holder.remove_reagent(/datum/reagent/tatp, created_volume)
 	..()

code/modules/surgery/organs/internal/liver/_liver.dm

@@ -142,7 +142,7 @@
 		for(var/datum/reagent/toxin/toxin in cached_reagents)
 			if(toxin.affected_organ_flags && !(organ_flags & toxin.affected_organ_flags)) //this particular toxin does not affect this type of organ
 				continue
-			var/amount = round(toxin.volume, CHEMICAL_QUANTISATION_LEVEL) // this is an optimization
+			var/amount = toxin.volume
 			if(belly)
 				amount += belly.reagents.get_reagent_amount(toxin.type)