Remove obsolete functions for handling Ragged-Array DSG data

[AnsleyManke]

The functions
FC_ISUBSET (Returns a subset of a feature collection)
EXPND* (various functions which insert gaps, or make a 2D version of data)
SEPARATE (insert gaps between features)

were written to handle DSG datasets largely for LAS. First deprecate and then probably outright remove these functions.

See issue #1169, which contains a hodge-podge of discussions about DSG datasets.