Added Mode, SingalInfo, and AlertFlag to output

Added a few more data types from the PVTGeo block.
Mode tells which PVT mode the system is in
SignalInfo is a bit mask that tells which *signals* were used in PVT.  This
  is sort of a proxy for constellations, as each constellation has it's own
  separate signals.
AlertFlag is additional codes
Added TravGPS & options to skip some processing

Added the marker 'PT04', the Traveler GPS marker, to the list of
known GPS data stores.
Added and cleaned up some of the options to select specific sub-
functions of the script.  Now, you can turn off each of the four
SBF output types individually.
Shifting offset now applied a time of calculation

Previously, the 'shift' value, which allows the xPPSOffset values to be
applied with an arbitrary shift with respect to the TIC time stamps, was
actually applied on import of the xPPSOffset data.  This patch applies
the shift between TIC and offset time stamps during xPPOffset
corrections.

Also, an option was added to turn on calculation and plotting of the
frequency depature.
Updated README descriptions