interpolatestoptimes() function continues to interpolate past known time values

[sablanchard]

The interpolatestoptimes() function continues to interpolate past known time values using the last value in the series. This results downstream in a travel time value of 0 between those stops. These consecutive nans at the end of a series should be removed and not interpolated.

incorrect:
input = nan, 2, 3, nan, 5, 6, nan, nan
output = nan, 2, 3, 4, 5, 6, 6, 6

correct:
input = nan, 2, 3, nan, 5, 6, nan, nan
output = nan, 2, 3, 4, 5, 6 (drop the last consecutive records with nan from result)

[sablanchard]

This was not an issue before this commit: 4c1a296 so it was introduced as part of the pivot method.

[sablanchard]

Fixed with #7