Fix inconsistent docstring formatting

[jlaehne]

• headline for each signal class file
• set signal class docstrings bold
• Correct inconsistent naming of docstring sections (Parameters, Notes)
• Correct inconsistent lengths of section markers
• fix error in docstring formatting mentioned in Docstring formatting in RTD #105, (closes Docstring formatting in RTD #105)

[codecov-commenter]

Codecov Report

Merging #107 (b212cb2) into main (047d975) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #107   +/-   ##
=======================================
  Coverage   95.69%   95.69%           
=======================================
  Files          11       11           
  Lines         581      581           
=======================================
  Hits          556      556           
  Misses         25       25           

Impacted Files	Coverage Δ
lumispy/signals/luminescence_spectrum.py	90.00% <ø> (ø)
lumispy/utils/io.py	100.00% <ø> (ø)
lumispy/signals/cl_spectrum.py	97.01% <100.00%> (ø)
lumispy/signals/cl_transient.py	100.00% <100.00%> (ø)
lumispy/signals/common_luminescence.py	100.00% <100.00%> (ø)
lumispy/signals/el_spectrum.py	100.00% <100.00%> (ø)
lumispy/signals/luminescence_transient.py	100.00% <100.00%> (ø)
lumispy/signals/pl_spectrum.py	100.00% <100.00%> (ø)
lumispy/signals/pl_transient.py	100.00% <100.00%> (ø)

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