Depends on https://github.com/dune-community/dune-xt-common/pull/191

This is not actually necessary. Adding a handler for std::exception also
hid the more specialized handlers provided by pybind11 itself.

also fix bindings.cc location

This would need some work bc. addbind_Matrix_Vector_interaction would add
a matrix_type() to these vectors twice (once for sparse, once for dense)...

* la.so -> _la.so, to allow for magic in __init__.py
* create module infrastructure in build tree via symlink during cmake