Files are duplicated in file set

[RasmusGOlsen]

When the FileSet reference is set in the file the file gets duplicated in the file set.

import pyEDAA.ProjectModel as pm


fileset = pm.FileSet("FileSetA")
file = pm.File("myfile.txt")
file.FileSet = fileset
fileset.AddFile(file)

for file in fileset.Files():
    print(file.Path)

output

myfile.txt
myfile.txt

[Paebbels]

I think this case is covered by code on dev branch: https://github.com/edaa-org/pyEDAA.ProjectModel/pull/54/files#diff-5ca1d531578afaf790f0e51937ffa4056bdf077d1601b209c2f33c69eeb46facR835-R836

def AddFile(self, file: File) -> None:
	if file.FileSet is not None:
		raise ValueError(f"File '{file.Path!s}' is already part of fileset '{file.FileSet.Name}' and can't be assigned to an other fileset.")

	self._files.append(file)
	file._fileSet = self

There is currently no explicit check on duplicates.

I think 2 changes are needed:

1. Adding a set to quickly identify duplicates in a fileset. Iterating a list is not so good. I might run a performance test before that change.
2. Adding a check also on the File.FileSet property so both direction of linking files with filesets is then protected from duplicates.