Proposal: Add a default unit option

## Issue

Currently there is no way to specify a `unit type` on a input/output and have it indicate to use the
current scene unit. An explicit `unit` from the corresponding `unittypedef` must be used.

It is possible with other meta-data such as `colorspace`.

## Proposal

- Allow for a "none" value to be supported for `unit` as with `colorspace`
- The meta-data can thus be saved to / or read from a document without having a specific unit transform specified.

The issue names no files, tests, or entry points. Start by locating the existing `colorspace` metadata handling and the `unit`/`unittypedef` definitions, then compare how metadata is saved to and read from a document. Done means `unit` accepts `none` without requiring a specific unit transform and preserves that value through document I/O.

Written by the indexing model from the issue text.