Some implementing of getTS option to help double ended searches.

This would make all calls to __generateProductStructures() also return
the "transition state structure", a list containing [merged_reactants, merged_products]
with the atom ordering consistent (needed for double-ended searches).

This is incomplete, but along the lines of what used to be implemented.

Abandoning for now in favor of group-additive single-ended approach
Get transition state distances from group estimates.

Atom labels were deleted once used in __generateReactions.
These are needed for the transition state, so they are stored
for now. These should probably be deleted at a later point.

The estimator is called in model.py after generating the
thermodata. The kinetics has not been determined yet, but
as the atom labels are only known when generating the reaction
the structures needed for transition states have to be stored
then.

Other corrections to DistanceData to append comments about the
source of the distances used.