Bins are a pain

[damonge]

Pasting this from a conversation with @amaurea

Constructing bin objects is tedious and confusing. An Nside argument is needed, and setting it incorrectly leads to confusing messages about requesting too high entries during mode-coupling matrix calculation. I also wish sensible defaults were provided for the weights argument. Ideally one would be able to say: bins = Bins(lmax=5000, dl=50) or Bins(lmax=5000, fsky=0.03) or Bins([[0,50],[50,100],[150,200],...])

I agree with the above.