Sensor hil incr

This is the first section of the sensor_hil branch rebased on master. This includes the CMSIS library and the C++ math library wrapper for the CMSIS routines. There is also an example app called math_demo. You can type math_demo test to run through the unit tests. In addition to the arm hw accelerated math back end using CMSIS, there is also a generic c backend that I wrote. This will be useful for SITL.