Nested template definitions and instantiations

[ansharlubis]

I started completing the nested template in the generic matrix example #2814. The implementation is still very messy and only handles one level of nesting, breaks with arbitrary level of nesting.

[certik]

I think that this is great. Thank you so much for this work. I don't see any major issue, so I am going to merge it. The rest we can polish later.