Common Grid Origin

[hbrincon]

Due to the sensitivity of VoidFinder to the choice of grid origin, it would be good to fix the origin to [0,0,0] for all void-finding (as discussed in UofR cosmology meetings).

Right now, changing the grid origin for SDSS DR7 from the minimum galaxy coordinates to [0,0,0] results in a negative buffer length calculated in _voidfinder.py. This stems from line 412, box = coords_max - grid_origin, which creates an array with negative elements and propagates into the buffer calculation.

If we want to enforce a common grid origin, one quick solution might be to assign the grid origin as the floor of the grid_origin user input, which should snap the grid to be aligned with [0,0,0]. Otherwise, we should find a working method to account for the sensitivity of VoidFinder to the grid.