Fixed bug in PWC code, added manufactured solution folder

1) Fixed a sign error in the weak form for the piece-wise constant
version of the code.
2) A folder containing the manufactured solution test for the new weak
form is added. The go_SA_Sn_lnr_ibp_MG code uses piece-wise linear
representation of the material property and source, while still adopting
the fully integrated-by-part version of the weak form, to avoid dealing
with the gradient of the material property and the source.