Allow for spaces in file names

[s-heppner]

Currently, the check_part_names() function is used to validate file names, not allowing spaces.

Thanks to @jkhsjdhjs who did the digging:

Validating the filenames via check_part_name() is wrong, as spaces are ASCII characters and thus allowed in filenames.
However, non-ASCII characters still need to be percent-encoded. Thus, the correct way would be to replace the call to check_part_name() by a call to a function that percent-encodes the passed filename, or at least to change the current validation.

This was found due to the bug reported in eclipse-basyx/basyx-python-sdk#236.