Use accession number in phylogenetic workflow

[kimandrews]

Multiple reasons have been identified to use the accession number rather than the strain name in the measles phylogenetic workflow:

• Many sequences have no strain name listed on NCBI
• Strain names do not follow a consistent format
• Some strain names are duplicated

[kimandrews]

Done in https://github.com/nextstrain/measles/pull/19/commits/0b3e4c160e999b3c0c820da0cc66d17b34b1d5ce