Feature/naik reconstruct

[maddyscientist]

This pull request fixes support for long-link reconstruction when using HISQ fermions with QUDA offload.

• When using HISQ fermions, the tadpole factor passed to QUDA needs to be set to unity due to how the HISQ links are constructed.
• To enable support for long-link reconstruction with non-zero Naik epsilon correction, QUDA needs to know the naik-epsilon value that is associated with a given link. While the multi-shift solver had access to this information through the ks_param struct, the regular CG solver and dslash calls did not have this: with this in mind I have augmented the fn_links_t struct to include the naik_epsilon value when the links are constructed. This is then passed to the MILC-QUDA interface.

I've tested this code with su3_hisq_rhmd, and I see complete agreement now between running with QUDA_MILC_HISQ_RECONSTRUCT=13 and QUDA_MILC_HISQ_RECONSTRUCT_18 (well to solver tolerance anyway). Both for the multi-shift solver and the CG used for psi-bar-psi at the end of the trajectories.

This pull request is dependent on matching changed being made in QUDA (https://github.com/lattice/quda/pull/720).

[maddyscientist]

Thank you @detar for the merge.

The default is to use no reconstruction, so it would have worked fine by default previously. What broke was if the environment variable QUDA_MILC_HISQ_RECONSTRUCT=13 was set, then the long link would pick up the wrong scaling. It now works as it should regardless of naik-epsilon value.