throw rather than abort in assertions

[Peter]

I had a mysterious bug/crash that after relinking against a debug libyat with assertions activated turned out to be an out of range in Matrix::operator(). So I needed to go through all matrices, which was tedious and tiresome. I realized it would've been easier if assert had thrown an exception instead, so one could frame down the buggy code.

We're already doing this in header files with macro YAT_ASSERT, and I think I'd prefer to have all those replaced.

[Peter]

note to myself: use gdb to trace down the bug