Sky background Simulation

For know we use a constant sky background, define in photon/s. This definition is enough for instrument performance studies because it ensure that we use exactly the same value than in the PRR documents.
For future survey simulation, we will need the to able more realistic sky background (and sky absorption), depending on the pointing.
The sky background class and the sky absorption already able to define a background from a spectra or a model.
This feature have be push in the rest of the code.
We also have to fine a similar way to define the instrument additional background.