[MixtureDensity] interface of ``multi_evaluate`` usable as in base class

o MixtureDensity.multi_evaluate can now be used as defined in base class
  but with additional features ``components`` and ``individual``
o need to change internals of pmc because `multi_evaluate` cannot take
  argument ``components`` and return log(q(x)) in general
  (for pmc that is ok because not calculated values are multiplied by 0)
o can now use ``multi_evaluate`` in `DeterministicIS`