adding ultima genomics platform

[yfarjoun]

....to the list of allowed values for the PL tag.

[jkbonfield]

Looks like a no brainer. Thanks.

@jmarshall Were you considering doing this at the same time as #651? This misses a history section update, but 651 has it so we could apply that first and then amend that line at the time of merging this and let both arrive in master at the same time.

I can do the work if you wish, or assign yourself if you wish to do the leg work.

[jmarshall]

Sure, I'll do the leg work on both.

On issue #648 @yfarjoun asked for a final sign off on this (and checked if any other tags were desired) from @ilyasoifer et al, and I was hoping for that confirmation before going ahead and merging this one.