Add warning if title, institution, etc are not in metadata

[dnowacki-usgs]

From the CF Conventions:

The general description of a file’s contents should be contained in the following attributes: title, history, institution, source, comment and references (Section 2.6.2, "Description of file contents"). For backwards compatibility with COARDS none of these attributes is required, but their use is recommended to provide human readable documentation of the file contents.

We should check for them and warn if they don't exist