Add categories kwargs - fixes #15

[JR-1991]

As highlighted in #15, the current version of EasyDataverse does not offer ways to add tags to data files. Thus, the add_file method of the Dataset class has been extended to accept categories as a keyword argument.

Please note that the add_directory function cannot accept such tags. Instead, it is best to directly edit the categories attribute of a File object found in dataset.files after adding a complete directory.

Closes #15

Example

from easyDataverse import Dataverse

dataverse = Dataverse(SERVER_URL, API_TOKEN)
dataset  = dataverse.create_dataset()
dataset.add_file(
        local_path="path/to/my.file",
        dv_dir="some/sub/dir",
        categories=["My Tag", "Another tag"],
        description="This is a description",
    )