Allow mutation strings to have comments at the end (if this doesn't work already)

[rambaut]

Suggestion - allow comments in mutation strings. I.e.: "del:21765:6 # spike:HV69-"

The # denotes the comment but perhaps it just ignores everything after the first whitespace and # is just a convention.

[rambaut]

Additionally, starting a mutation string with a # should comment out the entire mutation