Resolve aliasing issue by renaming `parse_sequence` argument
Resolve aliasing issue by renaming `parse_sequence` argument

Renames the `fix_dates` argument of `parse_sequence` to
`fix_dates_format`. This name is a slightly more accurate description of
the variable and also prevents the overwriting of the `fix_dates`
function by a function argument.

Fixes #873

nextstrain/augur,nextstrain/augur,nextstrain/augur