Add stash translation for '2metre dewpoint'.

This is a oddball case, where the stashcode determines the height coordinate
-- exactly analagous to '2m temperature' (which we already have).
Installed in the same way as that one, it then requires an **additional** fix in "_um_cf_map.py", to provide the standard_name + units.
This may well be compounding a mistake, but I think a consistent approach is still the least confusing option.