Update submodule matrix to latest Tue Aug 11 00:39:55 UTC 2020

    - matrix in PX4/Firmware (747a909): PX4/PX4-Matrix@0fd99c5
    - matrix current upstream: PX4/PX4-Matrix@25c0455
    - Changes: PX4/PX4-Matrix@0fd99c5...25c0455

    25c0455 2020-08-10 kamilritz - comply with Firmware style requirements
f981cea 2020-08-09 kritz - add possibility to iterate over SparseVector data at runtime (#143)
e714a28 2020-08-07 kamilritz - use size_t instead of int in sparseVector
1869941 2020-08-07 kritz - SparseVector (#140)
6ed5dbc 2020-08-05 kamilritz - Construct Vector from Slice<1,M> alias row()
a126be0 2020-07-14 Matthias Grob - attitude test: refactoring to avoid identity quaternion confusion
572bafb 2020-07-14 Matthias Grob - Quaternion: use template type for division by two
242b38e 2020-07-14 Matthias Grob - Quaternion: remove deprecated axis_angle conversions
7a3009f 2020-07-14 Matthias Grob - Remove asserts for unsigned integer >= 0