Feature request: acceleration output in Earth coordinate system

[artur-iluk]

In blackbox_decode the acceleration out in done in controller coordinate system. It would be useful to have possibility to output accelerations in the Earth coordinate system, with gravity removed (az-1g). It is done in what way in blackbox_render, so it should be quite simply. All functions already exists in the code blackbox_render. It is just a matter of output these values to .csv file by blackbox_decode.

Proposed command line switch for blackbox_decode: --acceleration-csys acceleration output coordinate system (controller|Earth), default is controller