Refactor some netcdf save code (#4301)

* Refactor onto a common multidim var routine.

* Review changes.

* Review change : fix docstring in _get_coord_variable_name

Co-authored-by: Bill Little <bill.little@metoffice.gov.uk>

* Review change: fix docstring in _create_generic_cf_array_var

Co-authored-by: Bill Little <bill.little@metoffice.gov.uk>

Co-authored-by: Bill Little <bill.little@metoffice.gov.uk>

Author: pp-mo