Move stats calculation to BaseFeatureAdapter

[cmdcolin]

This makes it so that base feature adapter can perform basic local stats calculation, removing the need for adapters like SNPCoverageAdapter, GCContentAdapter, and BigWigAdapter to all implement this independently, and enabling anything that returns a feature score to have this (can also leveraged by FromConfigAdapter in nextstrain demo)

[cmdcolin]

Split out from gwas branch

[codecov]

Codecov Report

Merging #1640 (66547c1) into master (80e4526) will increase coverage by 0.10%.
The diff coverage is 95.40%.

@@            Coverage Diff             @@
##           master    #1640      +/-   ##
==========================================
+ Coverage   58.60%   58.70%   +0.10%     
==========================================
  Files         444      445       +1     
  Lines       20385    20378       -7     
  Branches     4787     4792       +5     
==========================================
+ Hits        11946    11963      +17     
+ Misses       8142     8118      -24     
  Partials      297      297              

Impacted Files	Coverage Δ
.../sequence/src/GCContentAdapter/GCContentAdapter.ts	1.96% <ø> (+0.64%)	⬆️
...ins/wiggle/src/LinearWiggleDisplay/models/model.ts	75.69% <ø> (ø)
plugins/wiggle/src/index.ts	100.00% <ø> (ø)
plugins/wiggle/src/WiggleRPC/rpcMethods.ts	51.42% <50.00%> (-2.63%)	⬇️
packages/core/util/stats.ts	95.91% <95.91%> (ø)
packages/core/data_adapters/BaseAdapter.ts	62.50% <96.00%> (+17.81%)	⬆️
...ments/src/SNPCoverageAdapter/SNPCoverageAdapter.ts	95.00% <100.00%> (+2.43%)	⬆️
plugins/wiggle/src/BigWigAdapter/BigWigAdapter.ts	96.87% <100.00%> (+2.04%)	⬆️
...inearGenomeView/components/RefNameAutocomplete.tsx	82.75% <0.00%> (-5.18%)	⬇️
plugins/wiggle/src/statsUtil.ts	88.67% <0.00%> (-3.78%)	⬇️
... and 1 more

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