Fix units and aggregation of length calculator

The units were in meters when kilometers were desired.
Also, the the mode aggregation was using an average when
it should have been using a sum.

Note: these number are now in the ballpark with reality,
but still don't look perfect. SEPTA rail is showing 551
kilometers, where it should be somewhere around 450.
Adding an issue to address this.
Merge pull request #152 from WorldBank-Transport/feature/fix-length-units

Fix units and aggregation of length calculator