Cf-pandas

Custom keys that were in the vocabulary but not in the dataframe were testing as present in the dataframe

Lots more new functionality has been brought over from cf-xarray!

Added merge and iadd to go with add.

Forgot to extend the v0.3.0 behavior to the accessor.

Small update to allow for splitting values by white space before comparing with regex patterns in `utils.match_criteria_key`.

Three new classes to help write and manage custom vocabularies.

The three classes are:
* Reg, class for writing regular expressions
* Vocab, class for writing and working with vocabularies/custom criteria/ontologies that will work with cf-xarray and cf-pandas
* widget class that helps a person sort through a lot of options to choose what should count as exact matches in a vocabulary, such as for a server they will use