- error message for not being able to write lockfile did not insert the filename into
  message: fixed by using proper f string
- changed all .format to modern f string in XDR.py
- add explicit test for offsets_filename()
- add additional pytest.warns() to tests to catch warnings (reduces warnings output and
  checks that all expected warnings are raised with proper messages)

Co-authored-by: Irfan Alibay <IAlibay@users.noreply.github.com>

…sis into fix-xdr-errormessages

- reduces warnings in test_xdr down to 2 (both related to del)
- test for empty selection and missing elements warnings
- rewriting more old-school parameter interpolation to f strings

Co-authored-by: Rocco Meli <r.meli@bluemail.ch>

…sis into fix-xdr-errormessages