Warn when skims are incomplete

[blakerosenthal]

Missing zone-zone pairs in skims and trip matrices are silently filled with zeros. This allows us to write files with manageable sizes by only including non-zero values in the trip and skim outputs. However, when re-using skims from previous runs, we don't check which zone-to-zone pairs were used to generate the original matrix. This means we might accidentally reuse an incomplete skim if the old run didn't include all the zone pairs from the new run.

If the old skim doesn't match the new zone list, we should rebuild the skim from the network.

Possible solutions:

• write mapping to the skim file and verify it matches/contains current run's zone list and max_dist

[blakerosenthal]

@bstabler Would be interested in your thoughts on this, but probably LOE ~2-4 hrs