Map.py inconsistent handling of metrics /

[jllanfranchi]

inconsistent treatment of metrics: args handled differently across chi2, llh, and metric* methods of Map.py

[philippeller]

Furthermore: in stats.py the checking of the input arrays and possible clipping of SMALL_POS numbers or inf is handled in every single function but should rather be done in one place (now is a lot of copy&paste code)

[LeanderFischer]

Ok, yeah, if this is still the case, then that should be tackled!