improved varlabel handling for pytplot (from ERG team)

[jameswilburlewis]

https://github.com/ergsc-devel/pyspedas_plugin/blob/master/ergpyspedas/util/tplot_vl.py

This code allows adding variable label annotations beneath plots, better matching the IDL behavior.

Currently, the ERG variant can be used by calling tplot_vl() rather than tplot(). Eventually, this feature should be merged into tplot() itself.

[xandrd]

The initial version (what you get if you call tplot_vl() instead of tplot() ) looks a little rough, with no vertical space between varlabel lines:

Suggestions to add optional parameters, and better defaults for:

• Choosing the font size other than default
• Controlling the spacing
• Controlling the format of the output (e.g., resolution after decimal point, date output). Can be done as a list of string format for each line.

Additional suggestions:

• Create a Jupyter Notebook showcasing how to operate with such plot.