[FIX] RESS: removed hardcoded values

[paulroujansky]

Description

• Replaced hardcoded value for fwhm when computing left covariance of noise
• Added peak_width to set fwhm when computing signal covariance (from Mike X Cohen)
• Enhanced test

[codecov]

Codecov Report

Merging #22 (a7a66f0) into master (1e8c7cf) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   74.53%   74.53%           
=======================================
  Files          18       18           
  Lines        2054     2054           
=======================================
  Hits         1531     1531           
  Misses        523      523           

Impacted Files	Coverage Δ
meegkit/utils/stats.py	39.71% <ø> (ø)
meegkit/ress.py	100.00% <100.00%> (ø)

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