Priority levels default to 3.  other fixes...

setPriority debugged and working.  Now host controller communications
take priority over gps communications take priority over navigation's
state checking takes priority over compass reading.
improved isDataValid function for GPS strings
Improved multithreadedness

Added several places where interrupts are temporarily allowed when
computing heading.
Removed a single warning ...
Merge branch 'master' of https://github.com/jcronq/Final-Project-v1.0 into I2C_test