Update submodule matrix to latest Mon Dec 9 12:38:11 UTC 2019

    - matrix in PX4/Firmware (f2e6e81): PX4/PX4-Matrix@a8009a3
    - matrix current upstream: PX4/PX4-Matrix@4f3565d
    - Changes: PX4/PX4-Matrix@a8009a3...4f3565d

    4f3565d 2019-12-09 kritz - Add asserts (#115)
2f63981 2019-12-06 kritz - Add several functions that are useful for coavriance matrices: (#114)
5cbcf60 2019-12-05 kritz - Set Matrix's col amd row to single value (#113)
ef442fa 2019-12-05 kritz - Getter function for the diag elements of slice (#112)
de6a2d3 2019-12-04 kritz - Slice assign value (#111)