* this is way different datetime strings provided by the user that have the same contents are stored in a comparable datatype

* reported by Andrea

* sometimes Levante is very slow

* could cause an error if the same file was read in twice on the same process

* in case spread distance was 0.0, the scaling type selected by the user was ignored

* renames core_mask to owner_mask in distributed directory

* adds checking of core mask for consistency
  * all edges of a valid cell also have to be valid
  * the two vertices of a valid have to be valid
* distributed grid does not contain any cells/vertices/edges that are
  masked out be the user provided core mask
* dist_grids's can now also contain hanging edges/vertices (not
  connected to any valid cell/edge)