Add support for specifying units for a column in metadata.json and rendering them on display using pint.

Example table metadata:

                "license_frequency": {
                    "units": {
                        "frequency": "Hz",
                        "channel_width": "Hz",
                        "height": "m",
                        "antenna_height": "m",
                        "azimuth": "degrees"
                    }
                }

Example result

This works surprisingly well! I'd like to add support for using units when querying but this is PR is pretty usable as-is.

(Pint doesn't seem to support decibels though - it thinks they're decibytes - which is an annoying omission.)

(ref ticket #203)