Refactor average cost reformatting for wait times

[kuanb]

Fixes https://github.com/kuanb/peartree/issues/95

Better/more clear handlings of Series conversion to DataFrame. Also, check for when no valid wait times are parsed. If this condition occurs, then raise a custom exception.

UPDATE: Do not raise exception. The issue is when there is just, say, one line with only on stop at each stop an no valid wait time for the given time period. Again, this is the case in the example GTFS. So, the average wait times dataframe is empty. This is OK! We just need to accept that this part is empty and proceed to fallback wait time assignment step.

Example GTFS that would trigger this: f-9rc-highdesertpointorus.zip

[codecov[bot]]

Codecov Report

Merging #96 into master will increase coverage by 0.08%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   91.71%   91.79%   +0.08%     
==========================================
  Files          12       12              
  Lines         881      890       +9     
==========================================
+ Hits          808      817       +9     
  Misses         73       73

Impacted Files	Coverage Δ
peartree/summarizer.py	92.35% <93.33%> (+0.42%)	:arrow_up:

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[kuanb]

Note, update added to original PR notes:

UPDATE: Do not raise exception. The issue is when there is just, say, one line with only on stop at each stop an no valid wait time for the given time period. Again, this is the case in the example GTFS. So, the average wait times dataframe is empty. This is OK! We just need to accept that this part is empty and proceed to fallback wait time assignment step.