List of files in an ELN does not work

If you define a str typed quantity with a ['*'] with the FileEditQuantity dropping and reading the file has problems. E.g.

files = Quantity(
    type=str, shape['*'],
    a_eln=dict(component='FileEditQuantity')

Ask @aalbino for details. See also #798 (closed)

Ideally this is added to the nomad/datamodel/metainfo/eln/material_library example and the corresponding test case.