--write-taxodium ouput changes for other pathogens

[amkram]

This PR introduces some changes necessary to build a Taxonium tree for M. tuberculosis.

• Multiple metadata files containing the same fields are handled correctly.
• Genes on the negative strand are translated properly. Note that genes with introns are not properly handled yet.
• --x-scale is added as an optional argument to matUtils extract --write-taxodium. This scales the x-values so the tree can be viewed at a desirable scale in Taxonium. This may be a temporary addition that is removed as Taxonium evolves -- I would appreciate @theosanderson's input on this :) The default is set to 0.02 for SARS-CoV-2 trees.

[theosanderson]

Wow -- it's super cool to imagine Taxonium being using for other species!

The x-scale thing is a good Q. Firstly I definitely need to add some X zoom, pan function, it's just been somewhat tricky and never top priority. But I'm not sure if that 100% addresses this. I think one may always need some way of manually specifying either the scale, as here, or else the initial zoom range for the x axis (which comes to sort of the same thing). This is because there can be outliers, so I think someone probably has to design what things look like initially (i.e. deciding what is an outlier and can be cut off). But i guess if you don't think that is the case then you have the option of automatically setting this value based on the tree.