The cell optical depths in HI, HeI and HeII were not declared
in photoion.
Removed spurious Makefile in code directory

Removed the files_for_3D/C2Ray_3D_test_periodic exectuable
from the repository
Removed executables which do not belong in the repository.
No real change but for now set mesh to 100, 100, 100 for the
standard test problems.
Removed compiled program C2Ray_1D_test from repository