Refactoring and further quality improvement on nexus/nxdl_utils code

[mkuehbach]

Currently pynxtools via core nexus utility functions within nexus.py make use of nxdl_utils.py from nexus_definitions. There are aspects of these two set of library functionalities that are worth a refactoring:

• [ ] Refactor the current use of a mixture of etree and lxml to use lxml throughout
• [ ] Refactor to assure that type checking of function argument and return values in called and calling functions

See related issue FAIRmat-NFDI/nexus_definitions#206