Support cf-python as 'mode' for opening product datasets

[sadielbartholomew]

At present Datapoint only supports the opening of datasets using xarray to xarray.Dataset objects, but we would like to add support whereby they can be opened by cf-python and provided each as a CF FieldList object of CF Fields.

There would be a way to configure to open with the reader of choice, which could be xarray still by default but with the option to swtich to cf-python, as already planned in the logic:

datapoint/ceda_datapoint/core/cloud.py

Lines 122 to 125 in 2bbf4ee

	if mode != 'xarray':
	raise NotImplementedError(
	'Only "xarray" mode currently implemented - cf-python is a future option'
	)

I don't have admin access on the repo so can't formally assign myself, but I will be working on this so please consider me assigned to this Issue!