Resolve units discrepancy in `water_use.py`

[timothy-nunn]

When the various water use variables are output they are described being m^3/day:

PROCESS/process/models/water_use.py

Lines 116 to 123 in bd4752b

	if output:
	po.ovarre(
	self.outfile,
	"Volume used in cooling tower (m3/day)",
	"(waterusetower)",
	self.data.water_use.waterusetower,
	"OP ",
	)

However in the data structure the units are listed as m3:

PROCESS/process/data_structure/water_usage_variables.py

Lines 44 to 45 in bd4752b

	waterusetower: float = 0.0
	"""total volume of water used in cooling tower (m3)"""

The correct units need to be established and the units need to be corrected so they are consistent.

[ChangeinX]

The correct unit is m³/day. In run(), rejected heat is in MW, equivalent to MJ/s. Multiplying by SECDAY converts it to MJ/day. Multiplying that result by volperenergy, measured in m³/MJ, produces m³/day.

I propose a documentation-only PR to update the WaterUseData descriptions for evapvol, waterusetower, wateruserecirc, and wateruseonethru, together with the two related wastetherm parameter descriptions. The calculations, output labels, and regression baselines will not change.

If you agree with this interpretation and scope, please assign the issue to me. I am happy to contribute the change.