Made pme_work_t opaque

Made pme_work_t opaque

Introduced pme_init/free_all_work to act on the newly opaque data,
and moved the helper functions that they call to pme-solve.c.

Renamed data type to pme_solve_work_t and field in gmx_pme_t to
solve_work, to help differentiate between the different "work"
structs.

Moved declaration of PME_SIMD_SOLVE to pme-solve.c, since it
is used nowhere else.

The PME work arrays are now allocated by their respective thread.

Change-Id: I02467ac2a4c2e8e6a9c45731ccec248b766609ff