The size_t type means a number of _bytes_, and it was misused as
counting e.g. the number of rows/columns in a matrix. Moreover, it is unsigned,
which can give strange bugs if a signed/unsigned mismatch occurs.
release of tvmet (inactive for 2 years and developer unreachable) as the
basis for eigen2, because it provides seemingly good expression template
mechanisms, we want that, and it would take years to reinvent that
wheel. We'll see. So this commit imports the last tvmet release.
