Improve DMNCompilerImpl class

[gitgabrio]

DMNCompilerImpl class is the core responsible of DMN model compilation, i.e. translation of model to "evaluators".

That class grown over time, and some methods became too big, missing some of the clean code principles.
At the same time, that class itself has a very limited test coverage, in isolation (its behavior is checked, indirectly, by other "integration" tests).

Scope of this ticket is to:

1. refactor the class to adhere to clean code principles
2. extend unit test coverage of the class.

(From https://www.geektrust.in/blog/2021/11/11/clean-code-series-part-3-clean-readable-code/)

Naming of variables, classes, methods, functions, etc. – Naming your variables, classes, and methods make a huge difference in the ability to read the code.
Keep It Simple, Stupid – Popularly known as the KISS principle, it helps in keeping the code as simple as possible so that it becomes more modular and maintainable.
Refactoring – Refactoring of code has to be a constant effort. The more often you refactor your growing code base, the cleaner it will be.
Well structured – Structuring a code is no easy task. It takes a lot of effort and experience to be able to structure your classes and methods in a way that’s more meaningful and extensible.

@baldimir @yesamer