NetCDF string length dimension name methods

[sadielbartholomew]

We noted it would be useful, and particularly after discussion arising related to NCAS-CMS/cf-python#69, to have methods that can get, set, delete and check for the existence of trailing string length dimensions.

With naming in line with methods in the existing API, the intuitive case would be for a set of four methods named nc_{get, set, del, has}_string_length_dimension.

[sadielbartholomew]

Transferring this Issue to cfdm since I've just realised all of the nc_ methods are defined there rather than here in cf-python (git grep "def nc_" | wc -l gives 0 in cf-python/cf but 79 in cfdm/cfdm i.e. in the codebase directories).