v0.4.2

0.4.2
* new kalman filter module wtih rc_test_kalman and rc_altitude examples
* new <rc/deprecated.h> to help some transition from v0.3 to v0.4 library
* convert all linear algebra from single to double precision due to rounding errors
* merge RobertCNelson's systemd service improvements
* add third order complementary filter function
* Improve DSM robustness
* rc_make_pid_file also makes the pid directory and provides helpful error messages
* 'make' in the library now runs on any architecture
* new rc_matrix_symmetrize and rc_matrix_subtract functions
* redundant motor functions removed
* Lots of random little bugfixes
* fix rounding errors in servo module