[Halld-offline] new position for 4th FDC package and updates to material maps

[Simon Taylor]

Hello, everyone.

I have made an update to the ForwardDC_hdds.xml file to reflect the new 
(final) position of the fourth FDC package, which has been moved 20 cm 
upstream toward the target in order to minimize the impact of the 
infamous 10 degree calorimetry hole.
I have also modified the material maps and the code needed to generate 
these maps, obtain values from these maps and perform calculations using 
the values from these maps by adding three additional columns that store 
pre-computed values needed for the multiple scattering calculations in 
the Kalman Filter.  The new material map for FDC4 is consistent with the 
new position.

Simon