New in Version 0.65 (compared to 0.62):
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- new function 'sgplm1' for spline-based GPLM estimation (just for
  1-dimensional nonparametric component functions)

- df (approximate degrees of freedom) added in 'kreg', 'kgplm'

- aic (Akaike's information criterion) added in 'kgplm'

- function 'backfit' added for backfitting