Means to read datasets from CDL as a string input #337

(Copied more or less verbatim from NCAS-CMS/cf-python#171)

We agreed it would be useful to support a way to read-in field constructs from CDL supplied as a string, rather than defined in a file, the latter of which is the only means currently supported. (Naturally CDL is the only format we support that can be provided as a string.)

This could be handy if, say, a user had CDL copied to clipboard and they wanted to work with it interactively with cf, since they could just paste it into a string in their Python session without having to save it to a file outside of the session.

An intuitive way to enable this would be a keyword argument such as cdl_string to cfdm.read to indicate, when True, that the input argument is a str of valid CDL rather than the path to a file, as standard, i.e:

cfdm.read(string, cdl_string=True)

where the default would of course be False.