Estimate frequency of fastest connections

[mpadge]

Estimate frequency of fastest connections

[mpadge]

That commit adds most of the functionality. It just needs to be switched on by:

• Exposing an extra binary parameter in m4ra_gtfs_traveltimes() to determine whether or not to do these extra calculations.
• When that is selected, add one line to call the gtfs_next_intervals() function.

And that should do it.

[mpadge]

Note that the above commit resolves the issue of some trip intervals being negative. Those are actually not a bug, but reflect instances where exclusion of initial trips leads to routing on subsequent trips which leave earlier overall, yet involve more transfers so are not picked up in the initial scan.

[mpadge]

Re-opening to incorporate the output of the GTFS function, which is part of the pre-calcualtion routines, into the output of the main multi-modal travel times functions.