Revert "changed all double to PetscScalar to make compiling FormOperators in complex mode possible"

This reverts commit 752f0bcfd8c55939be7114d38faab334c42c4466.

This would require more changes to work. I would need to go back to CodesForLibrary and change all double to PetscScalar, and I feel guilty doing that because XD wrote the code only for real. Not to mention there are like 10 .c files in CodesForLibrary.

I guess I will stick to either solving the block complex problem in slepc or doing it in matlab (without using the GUI; just a script)