[dune-pdelab] New istl MatrixBackend considered less user friendly

[Oliver Sander]

Dear pdelab,

I just switched from the old ISTLMatrixBackend to the new istl::BCRSMatrixBackend.
Everything appears to work nicely, but I find the new implementation more cumbersome
to use.

Before, I could simple instantiate GridOperator with the correct matrix
backend type, and the corresponding object was created automatically for me.
With the new interface, I have to create a backend object myself (that's acceptable),
and additionally I have to give it information about the expected matrix density.
But I am solving small problems on small grids.  Memory consumption and wall-clock
time are not important to me.  Therefore I don't want to be forced to think about
matrix occupation patterns.  Is it possible to make the backend object default
constructible again (maybe choosing 3^d as a default value), so people like me
don't have to think about issues that only affect HPC projects?

Thanks,
Oliver


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