Subsampled coordinates (1) and compression refactor

[davidhassell]

In CF-1.9, lossy compression by subsampled coordinates was introduced, and needs to be implemented in cfdm.

This a "new" type of compression for cfdm, insofar as the compressed data does not have a reduced number of dimensions.

It makes sense for there to be one compression framework, so implementing lossy compression by subsampled coordinates will entail a refactor of the exiting types of compression by convention (gathering and ragged arrays).

Reading, representing, and reconstituting in memory are covered by this issue. Writing coordinates compressed by subsampling to netCDF files will be treated separately.