[FIX-#861,DOC] fixed eigen 3.2.1 problems with GammaDistributionFitter

 * redefined gamma distribution to be 0.0 if parameters are <= 0.0, while this
   violated basic assumptions on statistical distributions it allows the usage of
   an unconstrained optimisation algorithm (like eigen's lm)
 * assigned myself as maintainer
 * updated documentation
 * fixed error in `GAMMA_DISTRIBUTION_FITTER_VERBOSE` code path
 * applied uncrustify

fixes #861