Have destructive and non-destructive versions

[ssiccha]

Revival of #25. We want e.g. versions EchelonMat and EchelonMatDestructive. Then the "core routine" DoEch.. can be destructive.

[jnkbrachter]

This is delayed for now. We want to have the following features:

• Destructive versions write the echolonised matrix into the input matrix
• The input matrix is refactored to be a blockmatrix and this is done in-place