parse user entries of SPEC files

xrayutilities spec backend does not parse user inputs marked by `#U` in the data file.
This will require a PR to the external package, see
https://github.com/dkriegner/xrayutilities/blob/main/lib/xrayutilities/io/spec.py
and for reference
https://certif.com/downloads/css_docs/spec_man.pdf#pdf%3Abm133