write more comments before lines in example notebooks

[pmcdonal]

E.g., in fisher_planck.ipynb, there is a line
fisher_clik = Fisher(likelihood_clik)
which takes quite a bit of time and looks like it might be doing derivatives, but at the end there is, e.g., no "precision" set so I guess not. Then the line
precision_clik = fisher_clik()
takes a longer time (printing nothing), where now I understand it must be doing the real calculation.
(I took the change to 3.044 out in case this was forcing recalculation, before realizing I could check that no "precision" was set before)

So I am curious what the first line was doing, but the more general comment is that a lot more comments in the notebooks would help really understand what is going on. (I'd like to just read through code path/step through in debugger... but I haven't had great luck with that)