Fix remaining lgtm hits, add badges

[jlaehne]

• Follow up on #131 for remaining hits.
• add badges to readme/documentation
• lgtm commenting should now be activated

Progress of the PR

• [x] Change implemented (can be split into several points),
• [x] added line to CHANGELOG.md,
• [x] ready for review.

[codecov-commenter]

Codecov Report

Merging #132 (2b52118) into main (7eed9e4) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #132   +/-   ##
=======================================
  Coverage   96.58%   96.58%           
=======================================
  Files          11       11           
  Lines         556      556           
=======================================
  Hits          537      537           
  Misses         19       19           

Impacted Files	Coverage Δ
lumispy/signals/cl_spectrum.py	98.30% <ø> (-0.03%)	:arrow_down:
lumispy/signals/luminescence_spectrum.py	92.77% <100.00%> (+0.04%)	:arrow_up:
lumispy/utils/axes.py	98.06% <100.00%> (ø)

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[lgtm-com[bot]]

This pull request fixes 3 alerts when merging 2b52118f6135c9384b7daa1bc703711661639415 into 7eed9e4e96e2e24379ceadb34a8349938b101338 - view on LGTM.com

fixed alerts:

• 1 for Unnecessary pass
• 1 for Unused import
• 1 for 'import *' may pollute namespace