write chapter describing unit handling

[mbjones]

Write a doc chapter describing units handling, including new features such as the deprecation ability and the linkages to udunits.

[laurenwalker]

Moving this to 2.1.0 since it is a lower priority field

[laurenwalker]

Moving this to 2.1.0 since it is a lower priority field

[robyngit]

@justinkadi reported that this is a feature PIs would make good use of, and currently citations are just added as text in the method fields. Here is documentation on the relevant EML field: https://eml.ecoinformatics.org/schema/eml-methods_xsd.html#ProcedureStepType_citation.

Steps:

• [ ] Implement an EML Citation model - must be able to parse and serialize an EML Citation
• [ ] Build a generic EML Citation Editor View
• [ ] Integrate the citation model & view into the editor

Note we have a Citation Model, but it's designed to parse a response from the metrics service. We'll probably need a new EML Citation Model, but we should consider whether we can combine these two.