max, min, and other single point return values

[ktritz]

Perhaps related to the slicing/point index issue, when max or min is used on a signal or axis, the value is returned still as a signal/axis type rather than a fundamental float:

nstx.s141398.mpts.te.time[-1]
Out[19]: 1.2483279705047607

nstx.s141398.mpts.te.time.max()
Out[20]: AxisMpts(1.2483279705047607)

We also might want to have a function that returns the axes point at which a max/min signal value is found.