Scalar C\ell accuracy test

[jmsull]

We did the power spectrum - we should do C\ell_TT,_TE,_EE (since there are now functions for those). I will take a first pass at this without digging too deeply into precision settings.

[jmsull]

This could be a separate issue but should also clean up naked factors of c hanging around the source functions...

[jmsull]

Started this - there is a test (with an inadequately coarse tolerance of 11%) here.
~10% seems pretty high given our agreement on P(k) and (at least nonnu) perturbations.

A first glance at this seems to me like we have a k-dependent issue that gets worse at higher k. Perhaps our k grid is not big enough? I would need to look at CAMB/CLASS LOS routines to see if we need to shift/expand our range.

Also updated the CAMB vs CLASS notebook to produce C_\ells using our existing setup (though I think I have a more recent CLASS version than Zack). Have not been super duper careful with CAMB but seeing agreement at 0.1% in (unlensed) TT,EE ratios after forcing reion taus to match. (TE is obviously crossing zero so will need to take another perspective there).