[VDIF_test.py, vdif.py] Polarization written and read from VDIF

I have now added the correct writing of the EDV pol bit.

0 : X or L
1 : Y or R

Thus the user can edit the VDIF_test.py file to set the pol bit
correctly for the particular experimental set up.

vdif.py will now read that bit out.