Implement Add/Remove parameter for core ops

[DocOtak]

This is a tracking PR while the work is done. This will start with adding tests for behavior before working on implementation. As of this first commit, the added test will fail since the implementation doesn't work yet

Here are some cases that need consideration:

• Add/remove of just an ancillary variable (flag, error, analytical temperature)
  • Addition function should be idempotent and not touch any existing variables or their data if they already exist
  • Removal function has bookkeeping to do in the attributes of the main variable
• How to deal with variables with an extra dimension (CDOM)
• What happens when the removal of a required (e.g. expocode) variable is attempted?
• Should the netCDF history attribute start to be populated, if yes, what should the contents look like?
  • Array of Strings?

[DocOtak]

This should have all the machinery needed to fix the S04P CTD file by remerging the BEAMCP submission back into the data. I'm going to attempt this before merging this pull request.

The function itself also needs some additional documentation and usage notes. Adding provenance attrs might also be worth exploring, but shouldn't hold up getting the data fixed.

[DocOtak]

The basic functionality is ready I think, the edge cases noted in the original PR text can be addressed when they come up as blockers or some other later time.