inspiral_lr: tweak ranking stat .__call__() interface

- instead of a dictionary of SNRs and keyword arguments of (SNR,
  \chi^2/SNR^2) pairs, switch to a dictionary of SNRs and a dictionary of
  \chi^2/SNR^2
- this is the first step toward re-defining the ranking stat as the ln L
  maximized over subsets of triggers.  this re-organization will simplify
  the iterations require to implement that feature