Update submodule matrix to latest Fri Oct 4 20:37:32 EDT 2019

    - matrix in PX4/Firmware (92a2061): PX4/PX4-Matrix@973999a
    - matrix current upstream: PX4/PX4-Matrix@92d1c87
    - Changes: PX4/PX4-Matrix@973999a...92d1c87

    92d1c87 2019-10-03 Julian Kent - More features: longerThan, norm, copyTo and slice on a Slice (#97)
740324c 2019-10-03 Matthias Grob - Make all array constructors explicit (#99)
7b34c1c 2019-09-30 Julian Kent - Enable branch coverage