Sptensor fixes

Some fixes in sptensor:

- accum correction for cases with more than one dimension:
    accum function was not behaving as expected when the number of dimensions in subs was more than one.

- Corrections in sptensor and test case to avoid test_unfold failure:
    with random initialization of test data, test_unfold was also failing randomly (only when cases with duplicated indexes are created). I added a seed initialization for a case where the test was failing and corrected the problems.

- Optimized sptensor creation in ttv_compute when result is vector-like. Also, in this case, the sptensor should be initialized only with nonzero values (also corrected).