Expose `add_quadratic_from_dense()` method for BQM and QM

There are times where it would be convenient to add quadratic biases from a large dense array to a BQM or QM. There is currently an internal method [`.add_quadratic_from_dense()`](https://github.com/dwavesystems/dimod/blob/1bfbd3c66641b0121d077fca2002a7c6cd0941be/dimod/binary/cybqm/cybqm_template.pyx.pxi#L273) that could be cleaned up and exposed via the python interface.