Make KroneckerLinearSolver a subclass of LinearOperator

[jowezarek]

Adressing issue #333, we make KroneckerLinearSolver subclass LinearOperator instead of LinearSolver in order to be able to use fast solvers of mass matrices of logical domains together with other LinearOperator objects.
This update further includes:

• methods creating fast kronecker solvers for 3D and 2D mass matrices, currently located in test_kron_direct_solver.py
• the removal of the BlockDiagonalSolver class. It did not provide any significant advantage over using a BlockLinearOperator instead.
• the removal of the DirectSolver(LinearSolver) class. It's two subclasses now directly subclass LinearSolver.
• In an attempt to unify the usage of linear operators, solve() methods of all LinearSolver subclasses as well as KroneckerLinearOperator.solve() don't take transposed as optional argument anymore. Instead, proper transpose() methods have been implemented creating new objects, as is the case for LinearOperator subclasses.