mutation_sheet: Write script to write a mutation sheet automatically from covidCG lists

[vicfabienne]

The mutation sheet needs to have the variants as columns listing the signature mutations (Amino acid seq location) but also the codon_number/location on the nt sequence.
Both is e.g derived by tables sourced from covidCG. For the first column some parsing has to be done, the other one is just the column "codon_num" .

It would be nice to have a script that parses together this mutation sheet from a given list of input files.

[jonasfreimuth]

Could you please clarify which data are downloaded from covidCG (e.g. the query) and the format the downloaded tables have?

[jonasfreimuth]

Also this is the same issue as in #68.