const static members 

I add definitions of CONST static member into the class definition of some classes so that the master can be compiled with clang++ (fix of issue #739)

The rule is that: 
- non-const static member should be declared outside the class definition. Otherwise, you get the following error: "ISO C++ forbids in-class initialization of non-const static member".
- It's however different for const static method. Indeed, "9.4.2/4 - If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions".

Even if the master can now be compiled with clang++, the following tests FAILED:
	 23 - testClone2 (Failed)
	 79 - testFMM (OTHER_FAULT)
	 89 - testIntegralInvariantMeanCurvatureEstimator3D (Failed)
	 91 - testLocalEstimatorFromFunctorAdapter (OTHER_FAULT)
because of a bad use of ConstAlias: 
e.g. in testFMM [ERR] Assertion Error - assertion (( false && "[ConstAlias::ConstAlias( T&& )] Const-aliasing a rvalue ref has no meaning. Consider Clone instead." )) failed in DGtal::ConstAlias<BallPredicate<DGtal::PointVector<2, int, boost::array<int, 2> > > >::ConstAlias(BallPredicate<DGtal::PointVector<2, int, boost::array<int, 2> > > &&): /home/roussillon/Git/DGtal/src/DGtal/base/ConstAlias.h(259)
Abandon

I created a new issue for that ( #742)