fix column slicing constructor

[ruairimoran]

Fixing the column slice constructor from another DTensor.

The following now works correctly

size_t nRow = 10;
size_t nCol = 2;
DTensor<real_t> d_mat(nRow, nCol);
DTensor<real_t> d_col1(d_mat, 1, 0, 0);
DTensor<real_t> d_col2(d_mat, 1, 1, 1);
std::vector<real_t> col(nRow);
std::iota (std::begin(col), std::end(col), 0);
d_col1.upload(col);
d_col2.upload(col);
std::cout << "col 1:\n" << d_col1;
std::cout << "col 2:\n" << d_col2;
std::cout << "mat :\n" << d_mat;