properties now return dictionaries rather than lists

[malmans2]

Closes #109

.axes, .coordinates, .cell_measures, .standard_names return dictionaries mapping standard name to variable names.

.get_standard_names still returns a list of standard names.

[dcherian]

Thanks @malmans2 Can you add a note to whats-new.rst. Another nice change would be to change describe to use these properties. That could also be left to a followup PR.

[malmans2]

I was thinking the same, I just have to add the missing keys to the properties. I'll do the it here!

[dcherian]

I think you could build the string for describe by looping over _AXIS_NAMES (for e.g.) and print the variable names when available in the property dicts.

[malmans2]

Should also .keys use .axes, .coordinates, and .cell_measures?

[malmans2]

Actually, is .keys the same as .standard_names.keys()?

[dcherian]

Should also .keys use .axes, .coordinates, and .cell_measures?

Yes lets do this.

Actually, is .keys the same as .standard_names.keys()?

See #105. Right now it gives all "CF keys". Not sure what the right choice is.

[dcherian]

Thanks @malmans2 Great to have you contribute!