3f5d18f5c5e79dbc8715d777456a1709f83a9090 dseries: allow assign to select by date string as opposed to dates

================================ DYNARE MAKE CHECK MATLAB RESULTS ================================ | TOTAL: 419 | PASS: 417 | FAIL: 0 | XFAIL: 2 | XPASS: 0

================================ DYNARE MAKE CHECK OCTAVE RESULTS ================================ | TOTAL: 417 | PASS: 415 | FAIL: 0 | XFAIL: 2 | XPASS: 0

A full log can be found on karaba in '/tmp/dynare-master-check-5CKtNxRvoM.log'.