Pipeline improvement : 2 new methods :
 - Get(std::string FilterInstanceName)
 - IsPresent(std::string FilterInstanceName)

use :
Mepp::BaseFilter::PtrSelfType Res = p.Get( readerFilter->ToString() ) ;

The method ToString is now available for each Filter type and give a unique string for each filter. This string can then be used to retrieve
the corresponding filter in the pipeline.

The current implementation is linear in the number of filters, but later
an std::map could be used to speed up the process.

Each filter class now have the 2 following methods :
static std::string GetClassName() {  return std::string("MeshFilter") ; }
std::string GetClassNameNonStatic() const { return GetClassName() ; }

The first one is a static method to be given to FunctorNew(){ Mepp::ObjectFactory::GetInstance()
        ->Register(GetClassName(), this); }

The second method is the instance version of the first one, with a polymorphism mechanism to allow the method ToString to work properly.