EN_RuleVariable contained EN_R_POWER which is not used/supported

[lbutler]

While looking at converting units for rules I noticed that the enum EN_RuleVariable included EN_R_POWER however it is not possible to use this in a rule.

I haven't tested the API yet to know what happens if you try to apply this variable in a rule but it won't work as expected.

Is a rule variable based on pump power output something we want to include or should it be removed?

https://github.com/OpenWaterAnalytics/EPANET/blob/506f0ecb9d9236166e306b9b29dc9f65a71edfa4/src/rules.c#L40-L57