Add definition of the KYNBB and KYNPHEBB model output for polarisation

according to the XFLT0001 keyword.

* Add definition of the KYNBB and KYNPHEBB model output according to the
  XFLT0001 keyword of the SPECTRUM extension of the data file.
* Speed up KYNBB and KYNPHEBB model when using the same parameters (e.g. when
  using several data sets of the same source) by remembering the output from
  the last computation.