Refactor the module

As @dzenanz pointed in #20 , the module needs to be refactored. Some classes seem to be specialized versions of classes that exist in the toolkit (Canny detection filters, morphological filters, etc.), but it should be investigated whether better inheritance can be achieved.

Some minor issues related to the refactoring, or which could be done while refactoring are:
- Separate the class header from the implementation (e.g. see [itk::DescoteauxSheetnessImageFilter](https://github.com/InsightSoftwareConsortium/LesionSizingToolkit/blob/master/include/itkDescoteauxSheetnessImageFilter.h)).
- Provide *Set* / *Get* methods using ITK macros for all ivars.
- Provide some default values for ivars (e.g. see [itk::DescoteauxSheetnessImageFilter](https://github.com/InsightSoftwareConsortium/LesionSizingToolkit/blob/master/include/itkDescoteauxSheetnessImageFilter.h)).
- Provide the implementation for the `PrintSelf` method in cases which is missing or incomplete.

- Prefer initialization lists over assignment.