This is the simplest workaround for a bug in the amg I'm hitting in
parallel tests in gdt.

[eigen-solver.internal.base] fix error messages

[container.common.matrix.dense] make axpy usable for other matrix types

Fix gdt compilation after CommonDenseMatrix change

Fix_issue_41

Merge branch 'python_infrastructure_update' of https://github.com/dune-community/dune-xt-la into cleanup_algorithms

# Conflicts:
#	dune/xt/la/container/conversion.hh
#	dune/xt/la/test/builder_definitions.cmake
#	dune/xt/la/test/compiles_totals.pickle
#	dune/xt/la/test/testruns_totals.pickle

DynamicMatrix is essentially a vector of vectors, so the memory layout is
not contiguous. We would like to be able to use algorithms from lapack and
blas, so I changed the backend to be a single std::vector plus index arithmetic