Update PLAID to better manage dependencies

[jordanpadams]

Current implementation

#15 will be implemented using a temporary solution where all discipline LDDs and the PDS common namespace are all in one uber JSON.

Proposed improvement

For a more refined solution, we would like to have each discipline LDD have its own JSON, and PLAID will parse that JSON for a new namespaces key to determine all dependent namespaces for the LDD, and load those other JSONs as appropriate.

Per this improvement to LDDTool, generated LDDs now contain this new key. See attached for the CART LDD: PDS4_CART_1G00_1950.JSON.txt

From this example you can see the new namespaces key:

[
  {
    "dataDictionary": {
      "Title": "PDS4 Data Dictionary" ,
      "Version": "1.16.0.0" ,
      "Date": "Tue Jan 19 19:36:45 PST 2021" ,
      "Description": "This document is a dump of the contents of the PDS4 Data Dictionary" ,
      "namespaces": ["pds:", "geom:", "cart:"] ,
      "classDictionary": [
...

PLAID should use this to then go through the other JSON files and parse each of those namespaces