BUG: sample/store surface field triggers dimension check (fixes #2361)

- when used for example with wallShearStress, the stress field is
  initially created as incompressible but later updated with the
  correct compressible/incompressible dimensions.

  If this field is sampled as a surface and stored on the registry
  the dimensions should be reset() and not '=' assigned, since that
  causes a dimension check which will obviously fail.