Make all Plotting Functions Return Handle to matplotlib.pyplot rather than Plotting

[engineerjoe440]

It seems to make more sense to return figure objects rather than plotting them directly. In this way, it becomes possible to modify the plots and extend functionality.

In many cases, this should be a simple substitution; for example:

[engineerjoe440]

This has been done for a number of the available functions:

• powertriangle
• convbar
• phasorplot
• receiving_end_power_circle
• sending_end_power_circle
• PowerCircle
• InductionMotorCircle

But more may need to be converted.