XPS reader: adding .slh metadata files for SLE reader

[lukaspie]

Fixes FAIRmat-NFDI/pynxtools-xps#3.

This is still an early draft.

[coveralls]

Pull Request Test Coverage Report for Build 7529012605

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 43.497%

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Totals
Change from base Build 7264459327:	-1.0%
Covered Lines:	4762
Relevant Lines:	10948

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💛 - Coveralls

[lukaspie]

Closed as this will be addressed in the xps reader plugin: FAIRmat-NFDI/pynxtools-xps#6