Need an easy-to-use wrapper for LMN transforms

[jameswilburlewis]

For FAC and MVA coordinates, we have fac_matrix_make and mva_matrix_make that operate on tplot variables,. and can be used with tvector_rotate to transform tplot variables to the desired coordinates. LMN isn't quite as easy....one needs to explicitly extract arrays from tplot variables (positions and solar wind data), pass those to pyspedas.gsm2lmn(), then turn the results back into tplot variables.

If we had a routine lmn_matrix_make that took the appropriate position and solar wind variables, and generated rotation matrices, it would be a lot easier to use and teach.