Add a function to pad regularly-spaced grids

[ivalaginja]

Not possible for irregular grids, but this would be very useful for regularly spaced grids.

[syhaffert]

I do have that code somewhere. Let me dig it up and I will start a PR for it.

[ehpor]

A small addition to this: cropping. The grid.subset() method is kinda meant to do that, but it's currently not smart enough to detect that you've selected a region which is still separated and so can return a Grid with SeparatedCoords. Right now, it always returns a UnstructuredCoords grid.

As a side question: what would be the arguments to this Grid.pad() function? I'd like to avoid anything to do with pixels, so it has to be physical distances in each of the axes.

[syhaffert]

Yes, I do use size units in my functions. So that should be okay. I might do this tomorrow because I have to figure out where I exactly used it....

[ehpor]

So exactly the same problem as me XD. I also have it somewhere, but I'll let you put in the PR.