Success! Can plot any arbitrary list of shotnums strs
Added averaging of VME signal from multiple shots.

* Takes the mean by iterating over different shots and then dividing
  by the number of shots.

* Implemented diag_params dictionary:
  Dict containing the parameteres associated with diagnostics.
       row: number of rows in VME file.
       cols: standard # of elements saved.
       ind: index within the VME file. In 'iv' files, ind=2 for current.
       vme: name saved under vme.

* Should be easy to switch back & forth between different diagnostics.
Use dict to implement to plot of any diagnostics

Implemented vme_plot_diagnostic which use the dictionary plot_diag_parms
to fill out the appropriate plotting variables for a given plot.

This will greatly simplify the plotting process over different parameters.
Modified vme_2diag_2d_plot and moved it to specific plot