Fix definition of moment magnitude Mw

Mw = 2/3 (log10(Mo)-9.05): fixed 9.0 and 9.1 to 9.05 in several places.

There are several other things that should be cleaned up regarding the units for rigidity mu and the fact that the Mw function of the subfaults class has different logic than in the faults class.

Note that the 9.05 factor is suggested by the USGS at
  http://earthquake.usgs.gov/aboutus/docs/020204mag_policy.php
although they write it differently since they assume mu is in units of dyne-cm = 1e-7 N-m.