Generates the Ewald field. It is done in parallel, the whole field is distributed among all cpus. The field is eventually saved in file grv_bhEwaldFName. In case grv_bhEwaldAlwaysGenerate is FALSE and the file with the Ewald field exists, the Ewald field is read from it. Ewald field is stored on a nested grid (with the highest density at small distances), one level of the EF grid is calculated by grv_bhGenEwaldFieldLevel which is called by this subroutine.