Clean up non-PME part of ewald module

ewald/ewald and ewald/ewald-util code had several different kinds of
stuff in it. Separated code that does Ewald long-range and
Ewald-family charge correction from the code specific to the group
scheme.

Moved general-purpose routines that are used in a few other places to
calculate Ewald splitting parameters to the math module.

Minimized header dependencies.

Removed unused things: FLBS FLBSZ

Removed use of typedef for existing opaque struct ewald_tab, per
policy in #1490. Renamed to gmx_ewald_tab_t.

Change-Id: I1394bbd02aa92e6581d011e52c5bee12406a0144