🚀 Feature: Change the DTIME name to GENCOST in the skim cores

[arashasadabadi]

It looks like that in Emme transit skims, DTIME is presenting generalized cost rather than drive access time only. In order to avoid further confusions, we are going to change the DTIME name to GENCOST in all skim cores. This needs the changes in tm2py scripts and all the UECs which use the DTIME.
Progress:

• Sufficiently defined
• Approach determined
• Tests developed

[DavidOry]

A related question is whether we want to include distance in the generalized cost formulation.