ENH: better default roi_func in map_to_grid

* Renamed `qrf_func` to `roi_func` in map_to_grid.
* Three build in `roi_func` build in, 'constant', 'dist' and 'dist_beam',
  'dist_beam' is the default for backwards compatibility.
* The 'dist' and 'dist_beam' built in RoI functions now take into
  account the offset between the nearest radar and the grid when
  computing the RoI.
* grid_shape and grid_limits are now required parameters to the
  map_to_grid function.
* grid_shape, grid_limits, and roi_functions have dimension ordered as
  z, y, x, which matches the output grid shape. Internal this order is also
  used.
* Better documentation for all of the above.
DOC: fixed map_to_grid docstring