Test and fix for mp_f19_decoy_discrimination failure

[roccomoretti]

The mp_f19_decoy_discrimination scientific test has a hard failure. It's crashing with the following issue in the minimization routine of relax.

core.scoring.hbonds.hbonds_geom: [ ERROR ] NAN occurred in H-bonding calculations!
core.scoring.hbonds.hbonds_geom: [ ERROR ] Dxyz                    -NAN                    -NAN                    -NAN  Hxyz                    -NAN                    -NAN                    -NAN

This PR pulls out the offending run/structure as an integration test.

Playing around with it, I figured out two things

1. The -constrain_relax_to_start_coords true flag is needed to show symptoms
2. It was specifically the derivative calculations which were causing NaN issues.

Inserting a number of std::isnan() checks and trying to trace back the source indicates the problem was MembraneGeometry::g_radius_gradient_dz(), specifically where it did a (p.x()-xo)/std::abs(p.x()-xo)), which results in a NaN if p.x() equals xo. We can rearrange this slightly, not only to avoid the NaN, but also to reduce the number of calculations needed. While I was at it, additional efficiency changes in MembraneGeometry were also made.