Adding logic to allow plotting "all" for thresholds and levels

[mkavulich]

Hi Gerard, sorry this is so late. Here is a description of the new logic:

• Users can now specify "all" as an argument for either level or threshold (or both).
• generate_metviewer_xml now returns a list of XML file paths rather than a single file path. If used in the original way, this simply means it's a single-item list, and the
• Returning a list of xmls allows the function to be called recursively, by "extending" the output list if there are already items in it
• If "all" is specified for level or threshold, it loops over the dictionaries set in the get_static_info function: valid_levels_by_fcst_var and valid_threshs_by_fcst_var respectively. It then recursively calls the generate_metviewer_xml function for each of these levels and/or thresholds
• Finally, at the top level we simply loop over each item in that list of xmls, calling run_mv_batch for each xml generated.

Let me know if you have any questions or there's anything else I can help with!