Fix assertion in SparseMatrix

[volkm]

Assertion was violated for empty matrix.

[sjunges]

Is it on purpose that I can now iterate over row 20 if there are no rows, but i cannot iterate over row 20 if there are 5 rows?

[volkm]

Good point. I revised the assertion.

[sjunges]

Thanks, but what is the use case to be able to iterate over a non-existing row?

[volkm]

In my view, it should be possible to create an empty matrix. But then the new test in this PR fails, because in the constructor we call updateNonzeroEntryCount which tries to iterate over all entries. My goal is to support the iterators for empty matrices as well, such that for example begin() != end() always works.

[sjunges]

I agree that begin() should work, but I dont know whether begin(0) needs to work?

[volkm]

I think we need to support 0 because it is the default argument for begin().

[sjunges]

I see. Then I am ok with this solution, but would personally have considered seperating the implementation of the functions without argument.

The comment about the "end" code still stands? As in, shouldnt one be able to also call end(0) now?

[tquatmann]

I also think that begin() and begin(index_type row) should be two different overloads. Then, begin(i) is only valid iff row i exists and its the same for end.

[volkm]

Thanks for your comments. I decoupled begin() and begin(row). This is also now consistent with end which already had both these functions.

[sjunges]

LGTM