Exosuit Nuclear Power Cores (#18268)

* Added power cores, a type of large battery cell that get used by
exosuits. The nuclear and phoron variants are self-charging.
* Combat mechs now start with nuclear power cores, allowing them to
sustain themselves indefinitely, so long as they stay out of the action
for a bit.
* Removed basic power cells from the mechfab, replaced with the mech
powercores.
* Mech cell statuses now instantly update as soon as the cell charges or
discharges.
* Added a stack of 10 uranium to the machinist's workshop, which can
print two nuclear power cores.

code/modules/power/cell.dm

@@ -4,9 +4,30 @@
 
 /obj/item/cell/Initialize()
 	. = ..()
+
 	charge = maxcharge
+
+	if(self_charge_percentage)
+		START_PROCESSING(SSprocessing, src)
+
 	update_icon()
 
+/obj/item/cell/Destroy()
+	if(self_charge_percentage)
+		STOP_PROCESSING(SSprocessing, src)
+	return ..()
+
+/obj/item/cell/process(seconds_per_tick)
+	if(self_charge_percentage)
+		// we wanna recharge [self_charge_percentage% of the max charge] amount every 60 seconds
+		var/recharge_amount_per_minute = (maxcharge / 100) * self_charge_percentage
+		// since process fires every ~2 seconds, we wanna get the recharge amount per second
+		var/recharge_amount_per_second = recharge_amount_per_minute / 60
+		// multiply the amount per second with how many seconds this tick took, then round it to prevent float errors
+		var/recharge_for_this_process = round(recharge_amount_per_second * (seconds_per_tick / 10)) // divides seconds_per_tick by 10 to turn deciseconds into seconds
+		// finally, charge the cell
+		give(recharge_for_this_process)
+
 /obj/item/cell/Created()
 	//Newly built cells spawn with no charge to prevent power exploits
 	charge = 0
@@ -59,6 +80,7 @@
 		return 0
 	var/used = min(charge, amount)
 	charge -= used
+	SEND_SIGNAL(src, COMSIG_CELL_CHARGE, charge)
 	return used
 
 // Checks if the specified amount can be provided. If it can, it removes the amount
@@ -78,6 +100,7 @@
 
 	var/amount_used = min(maxcharge-charge,amount)
 	charge += amount_used
+	SEND_SIGNAL(src, COMSIG_CELL_CHARGE, charge)
 	return amount_used
 
 
@@ -180,6 +203,7 @@
 	charge -= maxcharge / severity
 	if (charge < 0)
 		charge = 0
+	SEND_SIGNAL(src, COMSIG_CELL_CHARGE, charge)
 
 /**
  * Drains a percentage of the power from the battery
@@ -196,6 +220,7 @@
 	charge -= maxcharge / divisor
 	if (charge < 0)
 		charge = 0
+	SEND_SIGNAL(src, COMSIG_CELL_CHARGE, charge)
 
 /obj/item/cell/ex_act(severity)