Implement migrator that removes obsolete slots from MassSpectrometry documents

[eecavanna]

In this branch, I implemented a new migrator.

This migrator processes every document in the data_generation_set collection. For each document, the migrator checks whether it is of type nmdc:MassSpectrometry. If it isn't, the migrator skips processing this document. Otherwise, the migrator proceeds as follows.

The migrator checks whether the document has any of the fields that are in a list of obsolete fields. If the document has any of those fields, the migrator checks whether the field is empty or not. If it is empty, the migrator deletes the field (using "emptiness" criteria specific to the kind of field). On the other hand, if it is not empty, the migrator raises an exception.

I designed this migrator to run after all 14 other migrators that currently exist.

Fixes microbiomedata#2027