Implement support for "ITU-R BT.2020" colorspace.

[KelSolaar]

A few notes:

• The conversion matrices and some of the transfer functions constants are computed with colour. I did not added a reference as there are no references atm in the codebase.
• The way transfer functions are implemented as of now, i.e. in the RGB_to_XYZ and XYZ_to_RGB definitions is not very maintainable which is fine assuming only a few colorspaces are implemented.
• The matrices of the various RGB colorspaces don't round trip properly, I would suggest keeping the NPM and have the inverse computed with np.linalg.inv.
• I added a round-trip test for the RGB colorspace with transfer functions in mind, could be extended but it is a start.

[gtaylor]

I don't know anything about this color space, but the code looks reasonable.

[KelSolaar]

@gtaylor : The TLDR version is that it is basically the successor of sRGB/BT.709, it is becoming more and more important.