grid_points improvements

[ryanlevy]

For certain values, grid_points will not get close to the value of 1.
For example, 50 grid points on L=6 will result with a max of 0.796875

Also a wishlist, it would nice to enforce a certain point (like I want a grid but make sure 0.5 is there)

[JoeyT1994]

Yeah I think the grid_points functionality should be massively fleshed out

To add to the wishlist we should also be able to sample randomly from the grid. For instance we could overload rand so that
Random.rand(s::IndexMap) randomly chooses a point on the d-dimensional grid defined by IndexMap