Merge pull request #63 from tamiko/fix_manifold_instantiations

Merge pull request #63 from tamiko/fix_manifold_instantiations

This commit reverts the quick fix in commit 982b0683.

The issue why we encountered unresolved references to
dealii::FlatManifold::FlatManifold(dealii::Point, double)
etc. was due to boilerplate instantiations of Triangulation::get_manifold
for cases with spacedim < dim (in order to make TriaAccessor happy).

This commit fixes the issue by using a nullptr-dereference in order to
return an invalid object - the specific code throws an error message before
that and should actually not be reachable at all.