Refactor semantic constants to enumerations

[sadielbartholomew]

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

cf-python/cf/functions.py

Line 3501 in 02e348c

return ("nested", "ring", "nuniq", "zuniq")

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:

cf-python/cf/constants.py

Lines 522 to 542 in 8c760f8

	# --------------------------------------------------------------------
	# Logging level setup
	# --------------------------------------------------------------------
	# For explicitness, define here rather than importing identical Enum
	# from cfdm
	class ValidLogLevels(Enum):
	DISABLE = 0
	WARNING = 1
	INFO = 2
	DETAIL = 3
	DEBUG = -1
	# --------------------------------------------------------------------
	# Controlled vocabulary for bounds combination options
	# --------------------------------------------------------------------
	class OperandBoundsCombination(Enum):
	AND = auto()
	OR = auto()
	XOR = auto()
	NONE = auto()

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.