Fix USA panel

[standage]

Previously the usa panel included 100 loci for which there was supposed to be allele frequency data for all 19 sub-populations in a mock population roughly matching demographics in the United States. Due to a bug in that code, some loci in the panel do not have allele frequency data available for all populations. Also, since the initial panel slated for evaluation on real data (beta) contains 50 loci, it makes sense to restrict the usa panel to 50 loci as well.

This update fixes the bug and limits the panel to the top 50 loci ranked by A_e.

Related to #55.

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• [x] Changes are clearly described above
• [x] Any relevant issue threads are referenced in the description
• [x] Any new features are tested (see docs/DEVEL.md for details)
• [x] Substantial changes are documented in CHANGELOG.md (see https://keepachangelog.com/en/1.0.0/)

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage   97.18%   97.17%   -0.02%     
==========================================
  Files          26       26              
  Lines         818      815       -3     
  Branches      151      150       -1     
==========================================
- Hits          795      792       -3     
  Misses         11       11              
  Partials       12       12

Impacted Files	Coverage Δ
microhapulator/panel.py	98.02% <100%> (-0.04%)	:arrow_down:

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