Use dates to join ATS positions and transmissions

[ericnewkirk]

In order to figure out which data substitutions are happening in the fix data we need info from the transmissions, but relating fixes to transmissions is complicated and imperfect. Right now we use the NumberFixes column in transmissions and the row number in fixes, but there are inaccurate values in the NumberFixes column at times that throw everything off. Using the dates is not always accurate either, but results in fewer errors.