attribute editor: support custom units

[mbjones]

It is rare when datasets are completely covered by the standard unit list in EML. In the attribute editor, we must support the use of custom units that are provided by the user and linked to an SI unit by a conversion factor.

[csjx]

We need to discuss two aspects of this:

1. Supporting custom units in the EMLNumericDomain model and the EMLAttributeView in the editor

2. Support for storing and reusing custom units that have already been created, either on a per-user basis or as a unit registry (this can be informed by the LTER units registry (units.lternet.edu)

[jagoldstein]

Many units we receive are custom. This is potentially a big time consumer for us as we anticipate submitters will write in to say that their unit is not in the list and ask what they should do. We can tell them to pick the closest one, but often they will say that none are close. We can tell them just to pick something, and then we'd have to go back and change them to custom units that they define in an email to us or in an attached doc.

Also, dimensionless is used often and should maybe stand out separately , rather than being buried in an alphabetical listing. Some contributors look for unitless and may not find dimensionless. It would also benefit from examples: "such as for percent or ppm".

[mbjones]

This issue arose again in the Arctic Data training, with a strong vote for the need to be able to create custom units for new data sets.