Documentation: clarify wrapper concept and improve documentation on files, ids, aliases, and exchange items

[theavuik]

A wrapper consists of a set of Java classes that connect OpenDA to a model.
For D-Flow FM, this connection is file-based: the Java classes read, modify, and write model input/output files so OpenDA can perform data assimilation.
The documentation needs clarification and extension regarding wrappers and subsequently files, ids, aliases, and exchange items.
Clarify what % mean, mdufiles are, etc etc.

Once this page is made, references in the existing documentation can be added, for example on the page about the D-Flow FM wrapper.

[erikpelgrim]

First version:

WrapperExplanation.txt