Refactor semantic constants to enumerations

In code review at https://github.com/NCAS-CMS/cf-python/pull/910#discussion_r2975606868 I noted that we could replace a tuple defining valid indexing schemes with an equivalent enumeration:

https://github.com/NCAS-CMS/cf-python/blob/02e348c5c84784e6a1613c3722a3c2ed1a0fd25d/cf/functions.py#L3501

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:

https://github.com/NCAS-CMS/cf-python/blob/8c760f82ed119cdda0ad041bc9deec49f3bc15d4/cf/constants.py#L522-L542

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:
   https://github.com/NCAS-CMS/cf-python/blob/02e348c5c84784e6a1613c3722a3c2ed1a0fd25d/cf/regrid/regrid.py#L20-L29
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.

Start by reviewing the tuple in cf/functions.py, the existing Enum definitions in cf/constants.py, and the valid regridding methods in cf/regrid/regrid.py. Survey the listed constants and repeated controlled-value sets, then determine a focused scope and whether definitions belong in cf-python or cfdm. Done means the agreed semantic constants use appropriate enumerations without breaking existing comparisons or interfaces.