Rename pytplot parameters tvar_name, new_name, new_tvar etc. to "newname" to match IDL

[jameswilburlewis]

For the time being, we'll support the deprecated parameter names and issue a deprecation warning.

Routines to modify:

PyTplot:

• [x] add
• [x] add_across
• [x] avg_res_data
• [x] clip
• [x] deflag
• [x] degap
• [x] derive
• [x] divide
• [x] flatten
• [x] interp_nan
• [x] join_vec
• [x] multiply
• [x] resample
• [x] spec_mult
• [x] subtract
• [x] tdeflag

PySPEDAS:

• [x] avg_data
• [x] deriv_data
• [x] tdeflag
• [x] tvectot
• [x] wavelet
• [x] yclip