Adds necessary (& removes unnecessary) includes in DcdCollision.

Also forward declares appropriate classes.
Removes indentation in namespace in DcdCollision.h
Remove indentation in namespace in DcdCollision.h
Merge branch 'fix/dcdcollision-includes' into 'master'

Fixes includes in DcdCollision.

+1: Vien Tran, haizhouW

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/697373