Refactor semantic constants to enumerations #942

In code review at #910 (comment) I noted that we could replace a tuple defining valid indexing schemes with an equivalent enumeration:

We agreed we should at some point survey the codebase systematically for further appropriate places to use enumerations to e.g. make comparisons against well-defined sets of values such as controlled vocabs more robust, clear and type safe.

We already have Enums applied well for a few cases:

From a quick look, some further good candidates across the codebase for using these are:

1. Also in the constants module as above: the CF cell methods set, and perhaps the keys from the cr_coordinates dict and the formula coordinate types (atmosphere_sigma_coordinate, ocean_s_coordinate, ocean_sigma_z_coordinate, etc.) both of which appear across multiple dictionaries there (which could in turn be converted to data classes, also an improvement);
2. The valid regridding methods, i.e. keys to:
   

   cf-python/cf/regrid/regrid.py

   Lines 20 to 29 in 02e348c

   	esmpy_methods = {
   	"linear": None,
   	"bilinear": None,
   	"conservative": None,
   	"conservative_1st": None,
   	"conservative_2nd": None,
   	"nearest_dtos": None,
   	"nearest_stod": None,
   	"patch": None,
   	}

3. (Upstream in cfdm*) the valid backends for reading and writing: https://github.com/NCAS-CMS/cfdm/blob/e6ebddb4a0f533f079f7f1ea0da2efdf6595c928/cfdm/read_write/netcdf/netcdfread.py#L1334-L1339 (for netCDF only) and https://github.com/NCAS-CMS/cfdm/blob/e6ebddb4a0f533f079f7f1ea0da2efdf6595c928/cfdm/read_write/netcdf/netcdfwrite.py#L5768 respectively.

* we should consider for all Enums defined whether it is best to define and import them in from cfdm, or have them in cf-python only.