LinearSolver is abstract, concrete Solve moved to MatrixInversionLinearSolver

LinearSolver is abstract, concrete Solve moved to
MatrixInversionLinearSolver.
All tests are updated to use MatrixInversionLinearSolver where
necessary.
BeamIn2DModel updated for all today's commits

Additional tests added to show that moments and angles are incorrectly
flipped if the beam orientation is changed.
Redundant tests/examples removed.
Other changes for breaking changes committed today.