Add possibility to use the GPUPolynomialField in the O2 propagator and make the O2 propagator compile on GPU

[davidrohr]

This finally makes the o2 propagator compile fully on GPU, even though using the GPUPolynomialField instead of MagFieldFast.
So far, only tested the compilation. This is not yet used. Next step is to port the TPC Refit onto the GPU, using optionally either the GPU track param / propagator or the o2 TrackParCov / propagator.

@mconcas : This should finally bring also the propagator on the GPU, but as is you cannot use it in the ITS code etc out of the box, since it relies on the setup of the polynomial field in the constant memory of the GPUTPCTracking. Working on providing a functional setup for you as well...