fix nema lat/lon parsing error to be correct decimal degrees

modified: Adafruit_GPS.h
modified: Adafruit_GPS.cpp

added new method: 
  double ll_nemaEndingWithCommaToDecDeg_d(char * nemaLatOrLon, char nsew);
which is now called from parse(...) and can be used for both latitudes and 
longitude conversions to float values in the correct range.

Former code resulted in floating point latitudes of 3217.1234 for example without handling the minutes as a fraction of a degree.
New code properly separates the decimal minutes (17.1234 for ex) and turns it into a fraction of a degree (divide it by 60);
Code tested with actual GPS tracking.