problem with pwr_spec

[jameswilburlewis]

See MAVENSDC/PyTplot#231

It looks like the "name" keyword was updated to "newname", but the references to "name" in the function weren't updated:

https://github.com/MAVENSDC/PyTplot/blob/matplotlib-backend/pytplot/tplot_math/pwr_spec.py

Also, there isn't a check for whether the tplot variable actually exists in this function, so it crashes when you give it a variable name that doesn't exist:

x, y = d[0], d[1]
       ~^^^
TypeError: 'NoneType' object is not subscriptable