Adding draw png subcommand

[lucblassel]

Goal

it draws an input alignment to a png file, one sequence per line and one pixel per character.
This can be useful to quickly visualize a large alignment where other viewers like bioJS or seaview might struggle.

Color choice

There are 2 different color schemes:

• Nucleotides: comes from the bioSyntax paper
• Amino Acids: adapetd from the Shapely colour scheme found here

The color scheme is selected from the alignment alphabet with the bioSyntax scheme being preferred in case of alphabet ambiguity.

remarks

I chose png as the command name but it might make sense to let the user decide the output format and rename the command to something like image.

[fredericlemoine]

Thanks @lucblassel !