Consider a specific schema field for unit

**Background**
In Search discussion on 2 March the following scope was discussed - [see document](https://docs.google.com/document/d/1O6ZhZgiJUdywCk0YGnWvZECnyiHC1krINXRYT38p1yc/edit)
What is the scope of Search?
1. agree on how we search for things with a given data model
2. agree on how we describe data models
3. agree on the data models themselves

The conclusion was that 1 and 2 are in scope. 3 is beyond scope for specific models but is facilitated by 2. Notably models defined in 3 would be recorded via 2.

**Issue**
The issue here concerns 2. The schema provided by /tables//info are how the models in 2 are made available.
The specific request is that the metaschema used for 2 should provide explicit handling of unit of measure in structured form.
The current metaschema is JSON-Schema. In Search examples to date unit have been embedded in $comment.
e.g.
```
{'age':{
"type": "number",
"$comment": "UNIT 'Years'",
"description": "Participant reference age"
}
```
Finding some way to include unit should be considered. Likely using an existing semantic standard for unit of measure.
The following is a conceptual illustration of a minimal change to accomplish this.
```
{'age':{
"type": "number",
"unit": "Years",
"description": "Participant reference age"
}
```

As a test case, the handling of unit should allow for automated use of the kind of age mappings illustrated in this [FASP Script](https://github.com/ga4gh/fasp-scripts/blob/master/notebooks/search/mappings.ipynb).

Start by reviewing the metaschema exposed through /tables//info, the linked Search discussion, and the FASP mappings.ipynb example. Determine an agreed semantic representation for units rather than relying on $comment. Done means the schema explicitly represents units and supports the described automated age-mapping use case.

Written by the indexing model from the issue text.