TODO

 Densities
 =========

* implement host friendly read routines for the variable values_on_grid:
  - read_values_on_grid_ordered(map, count, stride) -> if use_default_ordering is False,
    then read the lookup table and return the desired slide in default zyx ordering.
  - read_values_on_grid_at(lookup_table) -> provide data for the given points only from
    the provided lookup table.
    
* implement host friendl write routine for the variable values_on_grid:
  - write_values_on_grid_at(lookup_table) -> need to be collective to broadcast internally
    the various sizes and create the start for each processor accordingly.

* complete the list<scalarfield> implementation in densities.c