Add try_into_nalgebra methods to convert to fixed-sized (and half-fixed-sized) nalgebra matrices

[elidupree]

Feature request is pretty self-explanatory.

I like nalgebra's compile-time guarantees about size (just for clean coding purposes), so it would be convenient to have a single-method conversion between them, rather than having extra steps to convert from the dynamic form.

When I searched, I found this page suggesting ways to do it; presumably it would be cleanest if the method fails anytime the size doesn't match exactly, rather than silently truncating, as I think fixed_slice and fixed_rows by themselves do. (Unfortunately I haven't had the chance to try it out yet, as I'm currently bogged down in unrelated version-compatibility issues.)

[vadixidav]

Sorry for the late response (a year and a half later 😅), but I believe the easiest way to do this right now would be to simply convert to a DMatrix and then go from the DMatrix to your constant sized matrix using fixed_view. This would allow you to get a fixed view of the DMatrix after conversion.

Of course, we should also be able to do this in nshare. I will leave this ticket open until we have this implemented. I do think we can add TryAsNalgebra, TryAsNalgebraMut, and TryIntoNalgebra to implement this. Note that the trait names are changing in the latest release. See #17.