Performance improvements

Extend supported use-cases for GPU version of update and constraints

GPU version of update and constraints can now be used for FEP, except mass and constraints free-energy perturbation.

Reduce time spent in grompp with large numbers of distance restraints

The time gmx grompp spent processing distance restraint has been changed from quadratic in the number of restraints to linear.

Issue 3457

Support for offloading PME to GPU when doing Coulomb FEP

PME calculations can be offloaded to GPU when doing Coulomb free-energy perturbations.

CPU SIMD accelerated implementation of harmonic bonds

SIMD acceleration for bonds slightly improves performance for systems with H-bonds only constrained or no constraints. This gives a significant improvement with multiple time stepping.