Proposal: Add a default unit option

[kwokcb]

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.