Update submodule sitl_gazebo to latest Fri Feb 22 12:38:23 UTC 2019

    - sitl_gazebo in PX4/Firmware (4e792b24): https://github.com/PX4/sitl_gazebo/commit/b34a96e7e5572209d21b9445be048c77ef041baa
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/2e80474653bcb7808a7dfced4cf403ef607d69da
    - Changes: https://github.com/PX4/sitl_gazebo/compare/b34a96e7e5572209d21b9445be048c77ef041baa...2e80474653bcb7808a7dfced4cf403ef607d69da

    2e80474 2019-02-18 Elia Tarasov - fix comment for mag strength table unit
9a9a137 2019-02-18 Elia Tarasov - fix comment for the file name
7c49b2f 2019-02-18 Elia Tarasov - add units to mag variables
c004d94 2019-02-18 Elia Tarasov - fix magnetic field calculation by using geo lookup table
e9a3940 2019-02-18 Elia Tarasov - fix comment on magnetic data source and properties
c76ac17 2019-02-18 Elia Tarasov - copy magnetic data tables and functions from ecl/geo_lookup
28f59c5 2019-02-18 Elia Tarasov - add constrain function
8c869ed 2019-02-18 Elia Tarasov - fix linker complain on multiple definitions
b5213a7 2019-02-18 Elia Tarasov - add getters for mag inclination and strength
d311233 2019-02-15 Elia Tarasov - fix D-component of magnetic field for Zurich according to WMM2015