fix license statements, add to user guide

[jlaehne]

Progress of the PR

• add license statement to user guide,
• correct license statements in python files,

[codecov-commenter]

Codecov Report

Merging #108 (29fa26a) into main (210a4b0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #108   +/-   ##
=======================================
  Coverage   95.70%   95.70%           
=======================================
  Files          11       11           
  Lines         582      582           
=======================================
  Hits          557      557           
  Misses         25       25           

Impacted Files	Coverage Δ
lumispy/signals/cl_spectrum.py	97.01% <ø> (ø)
lumispy/signals/cl_transient.py	100.00% <ø> (ø)
lumispy/signals/common_luminescence.py	100.00% <ø> (ø)
lumispy/signals/el_spectrum.py	100.00% <ø> (ø)
lumispy/signals/luminescence_spectrum.py	90.05% <ø> (ø)
lumispy/signals/luminescence_transient.py	100.00% <ø> (ø)
lumispy/signals/pl_spectrum.py	100.00% <ø> (ø)
lumispy/signals/pl_transient.py	100.00% <ø> (ø)
lumispy/utils/axes.py	96.85% <ø> (ø)
lumispy/utils/io.py	100.00% <ø> (ø)

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