First sketch of banded matrix implementation.
Renamed class and fixed typo
Add CellIndirection
Draft axpy kernel implementation
Renamed "to" -> "row", "from" -> "col"
Add more properties to cell indirection

Separate horizontal and vertical extents as well distinguishing between
"cell" and "facet" dofs.
Merge branch 'vertical' of https://github.com/firedrakeproject/firedrake-helmholtzsolver into vertical
Added correct import statements
First implementation that runs (but has not been tested)
Bug fix in indirection map.
Implemented UFL form assembly
Simplification: do not exploit block-structure

The code becomes significantly simpler if we do not exploit the block-structure of the matrix, but just store it as a banded matrix over all dofs. This will also allow usage of LAPACK routines for the banded LU decomposition.
Bugfix: banded matrix parameters were not set correctly
Cosmetic changes: renamed variables and streamlined code
Fixed bug in axpy.

To correctly carry out the axpy operation, permute the indices correctly when inserting the values of the LMA into the banded column matrix.
Added tests for banded matrix class
Implemented matrix-matrix multiplication
Added test for matrix-matrix multiplication
Remove unused method

This method was only added for testing
Added matrix-matrix addition

Implemented method for calculating C = A+omega*B