bug in pwr_spec

[ericthewizard]

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

[ericthewizard]

Another issue I noticed: 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

[xandrd]

@jameswilburlewis, since the plan it to "fork" tplot into pyspedas, let make sure that we fix this before.

[jameswilburlewis]

Thanks for letting us know! Should be an easy fix.