Fix a memory leak when destroying the IndependentMultimodal object. (#109)

* Fix a memory leak when deosrtying the IndependentMultimodal object. Ensure internally used objects
stored in arrays are destroyed correctly.

* package_bgs/IMBS/IMBS.hpp
  (Bins, BgModel): Add initialize function and destructor to cover the lifecycle of its member variables.

* package_bgs/IMBS/IMBS.cpp
  (BackgroundSubtractorIMBS::initialize): Use the new Bins and BgModel initialize functions.

* Whitespace-only fix.

* package_bgs/IMBS/IMBS.hpp
  (~BgModel, ~Bins): Cleanup whitespaces.