foucalc: Fix bug in usage of rate in input struct

* Previously, foucalc required the strict format struct{time, rate1[, rate2]
  as input. This was rather annoying, because often lightcurves have
  the colums {time, rate, error}.
* Commit 56dfa43d implemented the usage of rate instead of rate1.
  However, this worked only for a struct with the fields {time, rate}
  or - if more fields were used - one had to set the noCPD qualifier
* This is related to the way how the foucalc code infers the numbers
  of lightcurves it should process. Because of the rigit structure,
  the code used the length of the struct minus 1. This crashes as soon
  as there are additional fields in the struct
* This commit only counts the number of fields with "rate" in their
  name to deduce the number of lightcurves to be processed
* Also disable support of "rate" usage for CPD calculation: it can be
  confusing, and the user should be required to use rate1 and rate2