[WIP]Fix MatrixRef::lbegin/lend

[devreal]

This PR fixes two issues with MatrixRef lbegin/lend:

1. Fixed compiler error
2. Fixed lend() in LocalMatrixRef (and thus MatrixRef) to not return a nullptr but a pointer past the end of the range (GlobViewIter::local() returns nullptr if pointing beyond the current unit)

[devreal]

The offsets are still broken and I cannot seem to figure out which offsets to use. Will look at it again tomorrow... Any help is appreciated, though.

[devreal]

@fuchsto I'm giving up here as it is not clear to me anymore which are global and local block coordinates and indexes. Also, I suspect that there are fixes lingering around in your branches. I'd appreciate a timely merge or a quick fix as the broken lbegin() on blocks is a road block for me right now.

[fuchsto]

@devreal Sorry, I don't get it: what's the original issue? Is the use case covered in the unit test you added?
I found a bug related to #418 and will fix it in a separate PR, not sure if it helps your use case.

[devreal]

Sorry, I don't get it: what's the original issue?

This PR was my attempt to fix Matrix::lbegin a month after I reported the issue in #418 but I got lost in the usage of CartesianIndexSpace (does it hold local or global indexes? pretty confusing for me) My original use case should be covered by the test case I added here.