Update submodule matrix to latest Wed Apr 8 12:39:43 UTC 2020

    - matrix in PX4/Firmware (222e063): PX4/PX4-Matrix@a328929
    - matrix current upstream: PX4/PX4-Matrix@d613055
    - Changes: PX4/PX4-Matrix@a328929...d613055

d613055 2020-04-06 kamilritz - Add more assignment operators for slices
976ada4 2020-03-21 Matthias Grob - Matrix: min max comments and test style
fa7153e 2020-03-21 Matthias Grob - Matrix: omit min max nan case with same result