3 versions of self-adjoint form implemented in 3D using deal.II

General naming convention of the C++ source file: go_SA_Sn_MG_XXX.cc

1) const_source_sigma : simple test code with constant distributed
source and cross-section.
2) pwc_source_sigma: code implemented for problem with piece-wise
constant distributed source and cross-section.
3) linear_source_sigma: code implemented for problem with piece-wise
linear distributed source and cross-section.

Note: the weak form for pw-constant and pw-linear is different. Detailed
write-up about the weak form will follow.

Even-parity Sn and Canonical SPn codes are included for benchmark
purpose.