[_tools/chain] bugfix

o corrected initialization of self._points in case prealloc=0
[pmc/proposal] bugfix in MixtureProposal

o evaluate(x) raised an error if x was such that q(x)=0 (because evaluate
  returns log(q(x)) and log(0) is not defined)
o fix: explicitly ask if q(x)=0 and in that case return -inf
[regularize] --> moved function regularize to _tools
[pmc] added pmc-algorithm

o added references [Cap+08], [Cor+12]
o implemented calculation of sample weights according to [Cor+12]
o implemented pmc-updates according to [Cap+08]