Add a 1D version of grid_coordinates

[leouieda]

Instead of adding the ability to generated 3D coordinates to grid_coordinates, it's more straightforward to add a 1D version of numpy.linspace that can take a spacing instead of a number, knows how to adjust spacing/region, and can return pixel-registered coordinates. This function is then used in grid_coordinates and can be used to generate n-dimensional coordinates if desired (by calling numpy.meshgrid).

Relevant issues/PRs:

Fixes #138

[leouieda]

Had a hard time naming this. Decided to use linspace since it does what we would have liked numpy.linspace to do. But using it this way can make it confusing in the code and also in user code (vd.linspace and np.linspace are very similar). Any suggestion for a better name would be very welcome.

cc @santisoler @LL-Geo @MGomezN @aguspesce

[leouieda]

How about line_coordinates for lack of a better name?