mbs can now handle units

[NicolettaK]

This PR modifies the Mbs module in order to:

• handle output units
• make K_CMB as default units
• do not generate FG maps when make_fg = False, even if fg_model is defined

[paganol]

Ciao @NicolettaK, can we merge this?

[ziotom78]

Hi @paganol, the code should be ok, but I waited before merging it because the modifications were not documented. It should be trivial to do so (just add a couple of sentences), I'll have a look.

[ziotom78]

I have had a look at the patch and decided that the change in the docstring should be enough. It would have been nice to extend a bit the short example at the beginning of chapter «Synthetic sky maps» in the User's Manual, but for the moment let's be satisfied with this.