Define grid size for different box shapes.

[ojeda-e]

As pointed out by @orbeckst, after initial refactor in #34 the grid is assumed to be square. The grid should accept at least rectangular shapes.

To do:

• Define the size of the grid by at least x and y.
  Suggestion: box_size = max(u.dimensions[0], u.dimensions[1])
• Add xfail for orhtorombic/triclinic box.

[ojeda-e]

I think we I can close this issue since we have
self.x_range = x_range if x_range else (0, universe.dimensions[0])
self.y_range = y_range if y_range else (0, universe.dimensions[1])

in base.py and respective tests with rectangular box sizes.

Do you agree @orbeckst @IAlibay @lilyminium ? Am I missing something here? Thanks.

[lilyminium]

If you add a rectangular grid test then yeah I think this is covered!