Add PBS genome-wide scan

[31puneet]

This PR would resolve #293

• Implements pbs_gwss(), plot_pbs_gwss_track(), and plot_pbs_gwss() with normed parameter support
• Here is an example notebook to see how the methods work link.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.97%. Comparing base (c2c4f73) to head (6f5a9ea).
⚠️ Report is 53 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1299      +/-   ##
==========================================
+ Coverage   88.92%   88.97%   +0.04%     
==========================================
  Files          55       58       +3     
  Lines        6257     6503     +246     
==========================================
+ Hits         5564     5786     +222     
- Misses        693      717      +24     

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[31puneet]

Hi @jonbrenas, @tristanpwdennis PTAL