omp parallel for in Conv3D and ConvGrad3D

Actually I haven't been using the modified Conv3D op, and it isn't tested in my test script. 
The test script just tests ConvGrad3D. ConvGrad3D doesn't do any BLAS calls.
I just changed Conv3D for consistency, but that was probably a mistake.
How about I get rid of the change to Conv3D?
I'll fix the c_compile_args issue.