Added class to convert gnss message from uavcan to uorb
Fixed heading covariance calculation and build errors.
Fixed bug with zero-sized covariance arrays
Fixed comments
Address Paval's comments regarding extracting matrix from uavcan msg, position covariance calculation, and _poll_fds_num
Using proper math library. Corrected speed variance calculation
Merge pull request #1111 from achambers16/uavcan_gnss

uavcan: bridge uavcan->uorb for gnss msgs