fix sanity check for underdetermined systems + add test case

[j-danner]

fixes issue #21 and extends test cases appropriately.

[j-danner]

Now A is padded implicitly with zeros if A->nrows < A->ncols. Only if inconsistency_check=1 we check whether the padding is zero in _mzd_solve_left, and abort early when this is not the case, i.e., the system is obviously inconsistent.
Also this implicit padding is now mentioned in solve.h, but I am unsure if it fits your usual coding style, or it should be put somewhere else. If so, please tell me where to put it, or adapt the PR yourself.

[malb]

Cool & thank you!