Combined row-wise and positional per-sequence annotation shading

c.a. 2017 a prototype shading model for combining hit score and posterior probability was developed based on data vis 'transparency' shading - where instead of applying simple alpha blending, posterior probability was encoded as the amount of desaturatation applied to a colour encoding of the hit score for hmmsearch results.

This 'hack' is perhaps generally useful, but a couple of things are needed:
- extend the AnnotationColourGradient api to support enabling/disabling and IO of these colourscheme definitions via annotation files.
- allow the colourschemes to be automatically applied when appropriate annotation data for sequences are available (e.g. detect posterior probability scores from hmm search results).
- explore whether this model can be reused for other types of data
- also explore if this model can be re-used for Consensus or Conservation shading.