Check signal standard is "none" when using non-BT709 colorimetry for HD

[philipnbbc]

One of the changes made to improve compliance with RDD 44 (ProRes in MXF) was to default to the "None" signal standard for HD video that doesn't have BT709 colorimetry (color primaries, transfer characteristic or matrix coefficients). This is because SMPTE 296M and 274M require (it should be) BT709. If it's not then it shouldn't be labelled as SMPTE 296M or 274M.

The code should be checked to see if there are video formats that don't follow this logic. Keep in mind that certain products / companies may have or had a different view on this and bmx should follow whatever they do.

[thomasheritage]

Hi, thanks for your contribution.
Per #114 this repo is now being archived. You might wish to contribute to https://github.com/ebu/bmx instead (issues will be enabled on that repo shortly).