[h52nx][converter] group several call to `_generic_path_getter`

Description / Goal

Several modifications to the Nexus output:

• add title
• move beam to an NXbeam and save it under NXinstrument

TODO list

• add title
• refactoring: group several dataset getter #75 (closed)
• add NXbeam class (under NXsample probably ?)
• keep compatibility with the previous beam thing
• add test

INFO (how to use...)

• close #75 (closed)
• a pre-release wheel should be done for it