the only reason dune-istl was required was that the spline class used
ISTL's tridiagonal matrix. Since the spline class moved into the core
(along with a more capable tridiagonal matrix), ISTL is no longer
required by opm-material.
The difference between the version number (1.0) and the label (2013.03)
is: The version is time-independent but shows the level of compatibility,
see e.g. <http://www.semver.org>, whereas the label is a marketing tool
which describes the project progress.