Expand attribute metadata

[fwkoch]

In addition to metadata already present, this should include:

• units
• null_value (if unspecified for float attributes, np.nan is assumed)
• default_value

[rowanc1]

[banesullivan]

What about including a dictionary of ancillary information where the value types aren't enforced?

This could lead to unexpected behavior on the user side when trying to retrieve this information but it would provide a way for users to tag on any additional information they desire.

[fwkoch]

Thanks @banesullivan - there's a separate issue for that exactly! https://github.com/gmggroup/omf/issues/20