Metrics renaming - Githubissues

Fixes for clarity of the different cost and freight delivery metrics:

Renamed metric "TKM" to "Mt-km"
Revised units for discounting factor and for Mt-km
Added "_Discounted" and "_Levelized" suffixes to metric names and units where appropriate. Metric values are un-discounted otherwise.
Code cleanup for the assignment of the affected metrics.

NREL / altrios