Update submodule matrix to latest Tue Sep 18 12:37:50 UTC 2018

    - matrix in PX4/Firmware (6a217407806bac38328cd12903ee3aff52df14b0): https://github.com/PX4/Matrix/commit/dc3af8097798881b9b2e72a029a3c6c10abd059b
    - matrix current upstream: https://github.com/PX4/Matrix/commit/30d5a794326f41286b387581f7c21f4bdb8fa696
    - Changes: https://github.com/PX4/Matrix/compare/dc3af8097798881b9b2e72a029a3c6c10abd059b...30d5a794326f41286b387581f7c21f4bdb8fa696

    30d5a79 2018-09-14 Bart Slinger - testing float/double with Type template
707e288 2018-09-13 Bart Slinger - explicit casting
480c5f1 2018-09-13 Bart Slinger - static assert M>=N. floats to Type, arguments as const reference
7495794 2018-09-12 Bart Slinger - test zero divisions
3f2d3cf 2018-09-12 Bart Slinger - fix a div/0 condition
983a3f0 2018-09-12 Bart Slinger - use Matrix and Vector class for A and tau
98b8e2d 2018-09-12 Bart Slinger - formatting
0009328 2018-09-09 Bart Slinger - least squares solver for MxN matrices using QR householder algorithm