Update PV Bus Voltage Set Point Convention

[ccoffrin]

As per discussion in #797, these changes are proposed for the next breaking release (but still open for discussion):

• PF methods will use generator voltage set points for determining the vm value (instead of buses)
• If you read in data where more than one generator on a bus that have different voltage set points, a warning is generated
• if you try to run a PF with more than one generator on a bus that have different voltage set points you get an error

The biggest change will be that in OPF (and related optimization models), these generator voltage set point values will become part of the standard solution data.

CC @iagochavarry, @frederikgeth