modifying operator parameters externally

[kostrzewa]

@marcus-petschlies I've now merged the quda_work and get_cvc_to_work branches in the get_cvc_to_work branch. Changes to operator parameters, including theta angles, can now be made via the function

tmLQCD_set_op_params

as discussed in #446 with @sunpho84

This also means that I've removed the invert_quda_direct_theta function which was just a (bad) proposal. So, except for the still inconsistent interface between invert_quda_direct and tmLQCD_invert, I think this should now work for our purpose.

[kostrzewa]

This means that it is sufficient to set up a single operator in the input file with the proper solver settings and then use tmLQCD_[get,set]_op_params to control the twisted mass as desired, as well as the theta angles.