Timetable day and date combined

[AlexandraKapp]

• when "date" option is chosen for timetable construction (e.g. get_timetable(gtfs, date=20200925) also add calendar.txt is scanned for valid entries
• necessary especially for feeds where two feeds are merged where one uses the "calendar" option and the other the "calendar_dates" option

@mpadge I adjusted the filter_by_datefunction, as I couldnt think of a usecase where the old version would be needed without the inclusion of the "calendar" entries. Though this could lead to different results for people who used the function before and which would now have a different behavior. Therefore I was considering to add a parameter "scan_day_for_date = FALSE" to the get_timetable() function that is then passed to filter_by_date.

[codecov[bot]]

Codecov Report

Merging #45 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #45   +/-   ##
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  Coverage   92.74%   92.74%           
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  Files          15       15           
  Lines        1226     1226           
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  Hits         1137     1137           
  Misses         89       89           

Impacted Files	Coverage Δ
R/timetable.R	100.00% <ø> (ø)

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