Time syncronization plugin

[vooon]

@mhkabir described main steps for time sync. But rather than doing that in sys_status i'll create new one: sys_time.

Also ROS API for gps will be changed, initially it copy nmea_navsat_driver package interface.
But in that release i don't spread usage on other plugins.

Related #155, #95.

[mhkabir]

Please note, offset is calculated as (this system) - (other system). Offset added to all message timestamps which come from PX4 side(time_boot_ms) to get them into ROS time.

[vooon]

Moved to 0.9.

[mhkabir]

Done in #159
Code is untested till now, need to run it on a MAV.

[mhkabir]

@vooon As per new changes on PX4 side, we do everything in usec (microseconds)

Can you please update all ms to us and only convert if required (e.g mtime.time_boot_ms*1000)

[mhkabir]

@vooon Can you check this please :

[ 94%] Building CXX object mavros/mavros/CMakeFiles/mavros_plugins.dir/src/plugins/sys_time.cpp.o
/root/catkin_ws/src/mavros/mavros/src/plugins/sys_time.cpp: In member function 'void mavplugin::SystemTimePlugin::handle_system_time(const mavlink_message_t*, uint8_t, uint8_t)':
/root/catkin_ws/src/mavros/mavros/src/plugins/sys_time.cpp:197:68: warning: integer overflow in expression [-Woverflow]
   const bool fcu_time_valid = mtime.time_unix_usec > 1234567890L * 1000000;
                                                                    ^
/root/catkin_ws/src/mavros/mavros/src/plugins/sys_time.cpp:198:54: warning: integer overflow in expression [-Woverflow]
   const bool ros_time_valid = now_ms > 1234567890L * 1000;

Just a warning but worth fixing/cleaning up.

[LorenzMeier]

The right constant for 64 bit integers is ULL (for unsigned long long).

[mhkabir]

This is finished off. The plugin is done + tested + merged.
Just waiting for PR to be tested and merged on PX4 side.

[vooon]

PX4 support merged.