Add Gridfinity insert base

[mmalecki]

I was wondering if you'd be open to accepting a pull request to add a Gridfinity insert base class for building just the profile, on which others could then build their own not-boxy inserts, and on which the GridfinityBox could then be refactored to build upon.

What I mean is facilities for building the following example:

In order to achieve the effect of generating just the base, I created a GridfinityBox and called just render_shell(as_solid = True) on it. I'm just looking for a cleaner solution here for folks who come after me. This also doesn't work with magnet holes, etc., for which I had to hack render_shell to return early before cutting, and go back to calling render()

[michaelgale]

Hi! Thanks for the suggestion on rendering just a base solid. I think its a good idea and I agree it would be better to implement it more cleanly than hacking the GridfinityBox class render methods. Let me think on this and see if there's a way of doing it more elegantly. It might offer the opportunity to clean up some other things in GridfinityBox.