Create submatrix view (reference and pipelined)

[rasolca]

The following is needed:

• sub-pipeline of matrix (Add Matrix::subPipeline{,Const} #898)
• arbitrary offset for distribution (Allow incomplete first tile/block in Distribution #901)
• sub-distribution from distribution (Add MatrixRef to take a sub-matrix from an existing Matrix #934)
• sub-matrix reference of matrix (Add MatrixRef to take a sub-matrix from an existing Matrix #934)
• sub-matrix sub-pipeline of matrix (lower priority)

more details: https://confluence.cscs.ch/x/hYBZHw

[msimberg]

@albestro identified a potentially high impact use case for the sub-pipelined sub-matrix in in the tridiagonal eigensolver GEMM optimization work. He currently gets a sub-pipelined full matrix for tasks and then in many cases uses only a small part of the full matrix. On one hand the unused tiles in the sub-pipeline get released as soon as possible and may only have a small impact of scheduling. On the other hand there may be a lot of unused sub-pipelines which all add dynamic allocations.

[rasolca]

There will be no impact after the clean-up, as only the relevant submatrix will be passed to merge.