Move the PME Ewald coefficients to the PME structure

Move the PME Ewald coefficients to the PME structure

Ewald coefficients no longer get passed every step to gmx_pme_do().
Instead, they are passed to gmx_pme_(re)init,
as they only change together with the grid size, not every MD step.

Change-Id: Iea75063cf060317e3f0fcfd147431520b6c83a90