get_bitinformation: want to skip a variable if dim missing or return None for it's information content along missing dim

[aaronspring]

LGTM! I think it addresses only the first part of @rsignell-usgs comment of

I'd like to specify the "y" dimensions in a list, e.g. dim = ['eta_rho', 'eta_u', 'eta_v'] and then use whichever one is found for a specific variable.

though.
If a variable in a dataset with dims 'eta_rho', 'eta_u', 'eta_v' only depends on 'eta_v', the code might currently fail when e.g. dim='eta_rho' is provided. We could discuss if we just want to skip that variable in this case or return None for it's information content along dim=eta_rho.
Nevertheless, this PR is self-containing, so not objections to merge this directly.

Originally posted by @observingClouds in #106 (review)