Consider supporting submatrices? (prototype included)

[EricCousineau-TRI]

I've made a quick prototype for being able to include submatrices. I posted it and some more explanation on the KDE Eigen Forum:
https://forum.kde.org/viewtopic.php?f=74&t=123881&p=375217#p375217

The prototype may be found here:
https://github.com/EricCousineau-TRI/repro/blob/29fe69e/eigen_scratch/matrix_stack.cc

If I have time at some point, I'll see if I can (a) incorporate it into the plugin, (b) post to the Eigen mailing list to see if someone has something better, and (c) see if there is a way to tie it into the existing initializer stuff to leverage expression templates and all that.

[EricCousineau-TRI]

Actually had a change to tinker around with something that is hopefully expression template-compatible:
https://forum.kde.org/viewtopic.php?f=74&t=140012

Unfortunately, I could not leverage intializer_list due to its constraints on homogeneity.