Added missing function's documentations

[wverastegui]

This PR added the documentation of the following functions:

• compute_densities
• compute_mass_values
• compute_mz_sd
• draw_rt_correction_plot
• draw_rt_normal_peaks
• extract_pattern_colnames
• get_num_workers
• increment_counter
• plot_normix_bic
• plot_peak_summary
• plot_raw_profile_histogram
• plot_rt_profile
• span

Closes #150

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.42%. Comparing base (6ac2542) to head (50ffc18).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #225   +/-   ##
=======================================
  Coverage   66.42%   66.42%           
=======================================
  Files          23       23           
  Lines        2740     2740           
=======================================
  Hits         1820     1820           
  Misses        920      920           

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