Added fetching of MaveDB mutation data

[Tom Goddard]

Allow fetching mutation score data from the Multiplexed Assays of Variant Effects database MaveDB. This will allow researchers who don't collect there own deep mutational scan data to visualize data collected by others if it is available for their protein of interest.

I want this capability before publishing the article on the ChimeraX mutation score capabilities so a wider audience can use the new tools.

Another reason to add this capability is to have a wider range of deep mutational scan data for testing ChimeraX visualization.

[Tom Goddard]

The MaveDB entries don't have short names for the scores. Sometimes the scores are described with a 10 word sentence. I've used these long score names since I don't see any other choice, although the make the score menu entries in the ChimeraX mutation tools very long. I had to fix several mutation tools to quote the score names and mutation set names in cases where they include spaces.

[Tom Goddard]

I improved the log output about the variants that ChimeraX ignores: multi-residue mutants, deletions, insertions and stop codons, and to also log a disclaimer that ChimeraX only handles single amino acid non-synonymous and synonymous mutations. Looking through a dozen MaveDB data sets suggests this is going to be a disappointing limitation since most of the data mostly consists of these ignored variants. This raises the priority of adding support for those currently ignored variants.

[Tom Goddard]

Done.

MaveDB entry identifiers are 00000001, 00000002, .... I allow fetching specifying the integer id without the leading zeros:

open 1 from mavedb

I've added documentation for mavedb fetch.