Move some Digitization code from FATRAS to Core?

During discussion with @pbutti we thought if some of the digitization code from the `FATRAS` package might better belong to `Core`. More precisely, from the Classes and files in `Fatras/Digitization` only the `UncorrelatedHitSmearer.hpp` depends on `ActsFatras::Hit`

All other files in that directory only depend on each other or on Core functionality, these are:
```
Channelizer.hpp ChannelMerger.hpp DigitizationData.hpp DigitizationError.hpp PlanarSurfaceDrift.hpp PlanarSurfaceMask.hpp
```
as well as
```
Channelizer.cpp DigitizationError.cpp PlanarSurfaceDrift.cpp PlanarSurfaceMask.cpp
```

So these don't depend on any of the FATRAS infrastructure.

Are there conceptual or design considerations for keeping this in FATRAS?