added signal_to_noise_threshold option to remove_noise function

[StSchulze]

• allows to define a signal_to_noise_threshold when using the remove_noise function
• default behavior is the same as before, i.e. the noise level can be defined either directly (key word noise_level) or by estimating the noise level after defining a mode for this, and default S/N threshold is 1.0

[codecov[bot]]

Codecov Report

Merging #192 into master will increase coverage by 0.06%. The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   83.98%   84.05%   +0.06%     
==========================================
  Files          32       32              
  Lines        3509     3524      +15     
==========================================
+ Hits         2947     2962      +15     
  Misses        562      562

Impacted Files	Coverage Δ
tests/main_spec_test.py	99.02% <100%> (+0.04%)	:arrow_up:
pymzml/spec.py	75.06% <75%> (ø)	:arrow_up:

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