Move and edit variable expressions section
Move and edit variable expressions section

After some thought I decided it would be worthwhile to have
a section dedicated to describing dependency injection since
that's rather central to the simulation framework.
I put it at the end just before the API section.
The content is an edited/expanded version of that first written
by @daradib.

UDST/urbansim,VladimirTyrin/urbansim,ual/urbansim,bricegnichols/urbansim,VladimirTyrin/urbansim,SANDAG/urbansim,VladimirTyrin/urbansim,UDST/urbansim,SANDAG/urbansim,waddell/urbansim,apdjustino/urbansim,waddell/urbansim,waddell/urbansim,AZMAG/urbansim,ual/urbansim,apdjustino/urbansim,bricegnichols/urbansim,AZMAG/urbansim,synthicity/urbansim,SANDAG/urbansim,synthicity/urbansim,AZMAG/urbansim,waddell/urbansim,synthicity/urbansim,apdjustino/urbansim,apdjustino/urbansim,AZMAG/urbansim,ual/urbansim,synthicity/urbansim,UDST/urbansim,UDST/urbansim,SANDAG/urbansim,VladimirTyrin/urbansim,bricegnichols/urbansim,ual/urbansim,bricegnichols/urbansim