ENH: increase precision for ensight field cache (#2999)

- attempt to minimize rounding in the cached time values
  since these are also used to re-populate the case files

STYLE: remove ancient handling of "meshes" entry

- was superseded by "geometry" entry in OpenFOAM-1912 and later.
  Now remove the transitional shim, which was in place for
  restart migration from 1906.