Fix warnings

[mortenfyhn]

I have some build warnings in the mavros and mavros_extras packages when I build locally with Clang, which I've attempted to fix.

I think some of these changes are uncontroversial, such as

• reordering constructor initializer lists to avoid "-Wreorder-ctor",
• initializing yaw and yaw_rate explicitly to zero in the setpoint_trajectory plugin, and
• fixing the probably incorrect gps_rate initialization.

There was also a warning where MissionBase::initialize(ros::NodeHandle *_wp_nh) upsets the compiler because it "hides the overloaded virtual function PluginBase::initialize(UAS &uas) because MissionBase inherits from PluginBase. I've avoided this by renaming MissionBase::initialize but you may want to solve it differently, or give it a different name.

After this, all that remains when I build are unused variable warnings, mostly about COV_SIZE which is only used in debug builds. I suppose this could be fixed with the casting to void trick (void)COV_SIZE or with [maybe_unused](https://en.cppreference.com/w/cpp/language/attributes/maybe_unused) (but that would require C++17). Locally I've silenced this with a few #pragma GCC diagnostic ignored "-Wunused-variable".

[vooon]

LGTM. Personally i prefer [[maybe_unused]], but for v2...

[mortenfyhn]

LGTM. Personally i prefer [[maybe_unused]], but for v2...

I agree, [[maybe_unused]] is much nicer than the (void) "hack". (A well-established hack but a hack nontheless.)

Anyway thanks for the merge!