Changes to access2nc.py:
 - added calculation of net radiation
 - convert units of soil moisture from kg/m2 to m3/m3
Changes to compare_access.py:
 - added extra quantities to be plotted
 - added print out of lag at maximum correlation
 - sundry other clean ups and enhancements
Changes to ncconcat.py:
 - check of Fc units as workaround for Tumbarumba 2012
   issue where Fc units are mg/m2/s