Refactoring in libndt v0.28.0

[bassosimone]

We already started this refactoring in #141 and #142. The purpose of this work is to have small classes that implement specific libndt behaviors. This will allow us to write in a simpler way unit tests for each class and progress more easily towards full code coverage.

The main motivation of this refactoring, thus, is to change the structure of the library so that it is possible to move faster in making changes to it with confidence.