Allow definition of unit for schema fields

[hevp]

In metadata schemas it should be possible to add the unit of the field. An example field would look like this:

        "s_dec": {
            "type": "number",
            "title": "Declination",
            "description": "",
            "unit": "Radians J2000"
        }

The unit can be shown on the form and on the landing page of a record.