measure2Port

      measure2Port    makes 2 port measurement of a and b waves
         [rec_tx1,rec_tx2] = measure2Port(vnakit,settings,ports)
         input:
             vnakit: object (the board),
             settings: vnakit settings object,
             ports: mapping dictionary,
                 possible keys: ['Tx1','Tx2','Rx1A','Rx1B','Rx2A','Rx2B']
                 possible values: [1, 2, 3, 4, 5, 6]
         output:
             rec_tx1,rec_tx2: [6,num_pts] recording vectors for rx1-6 with
                              tx1 and tx2 as transmitter respectively.