Added checks on output vector in mxv

[GiovaGa]

Resolves #400

[anyzelman]

The MR fixes it for reference and reference_omp, but I thought you mentioned all other backends also run into this issue @GiovaGa ?

[GiovaGa]

You are definitely right. I have now fixed also for the nonblocking backend.
The other backends call the same internal function of the reference backend, so this should do it for them

[anyzelman]

Running CI, running all unit & smoke tests with LPF. Looks ready to merge if these are both OK. Concept release notes:

Prior to this MR, calling mxv or vxm with a dense descriptor while the input/output vector was not dense, would not yield ILLEGAL for all supported backends. This MR fixes that, and also ensures that ILLEGAL is returned when one of the vector masks is non-empty (size larger than 0) and sparse. Furthermore, this MR adds a unit tests to guard against regressions.

Thanks to @GiovaGa for spotting the bug and providing the fixes for the reference, reference_omp, and nonblocking backends!

[GiovaGa]

Seems good to me.
My only observation is that it may make sense to add an assert in exec_tests to guarantee that indeed at least one vector is not dense, as going quickly through the test, this doesn't seem obvious (and the function exec_tests does not specify such precondition)

[GiovaGa]

Right now, the test fails when running with 16 processes (and only in that case)