268 Provide grb::set(Matrix out, Matrix mask, Matrix in)

[aleksamilisavljevic]

Closes #268

[byjtew]

Nice job, as for the unit-test, you could rely on the matrix factories that we have.
e.g: L masked by I should return I
L being a Lower triangular matrix
I being an identity matrix

[anyzelman]

@aleksamilisavljevic @byjtew -- apologies, I thought this one was merged ages ago but found it still open just now. I've rebased it (and weirdly enough indeed a component from this MR was already in utils?) and will look at it more shortly: at the moment it builds but not sure if it passes unit tests, plus I see a spec for the missing primitive should still be provided). I'll take those remainder, hopefully small, steps over the coming days:)

[anyzelman]

It's back up with unit tests passing, still a few TODOs:

• check if it makes sense to unit test non-void to void, no-casting vs. casting, and void-to-void. At present, the unit test goes for non-casting and non-void to non-void only
• unit test should check passing empty masks
• BSP1D and hybrid implementations are missing
• reference_omp variant is not yet fully parallel

[anyzelman]

Upgraded unit test -- running the full suite before continuing some other time

[anyzelman]

All tests pass at this point, by the way (test results from over the weekend and including for LPF). Only one TODO remaining, which is the parallelisation of this function in the reference_omp backend.