per-symbol value calculation for positional alignment score colourschemes

Alignment colourschemes that colour each position of each sequence according to some process suffer from the problem that they cannot be used to compute a colour for a particular symbol that can be used to render each symbol in a sequence logo derived from the alignment's sequence profile.

A heuristic could be implemented to average over scores or applied colours in order to compute a reasonable colour for each symbol present in an alignment column. This could be done in a generic way, enabling sequence dependent colourschemes to behave more like simple zeroth order colourschemes.