[ENH] Adds ability to query gene groups

[rmarkello]

Related to #54 (doesn't close because we could add other groups).

Adds ability to get lists of specific "groups" of genes (i.e., brain, neuron, synaptome) etc from Burt et al., 2018, Nature Neuroscience.

Just including the CSV file in the distribution because it's pretty tiny (131KB) 🤷‍♂️

[codecov]

Codecov Report

Merging #83 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage   89.03%   89.13%   +0.09%     
==========================================
  Files          29       29              
  Lines        1423     1436      +13     
==========================================
+ Hits         1267     1280      +13     
  Misses        156      156

Impacted Files	Coverage Δ
abagen/datasets/__init__.py	100% <ø> (ø)	⬆️
abagen/datasets/datasets.py	93.18% <100%> (+1.75%)	⬆️
abagen/tests/test_datasets.py	100% <100%> (ø)	⬆️

---

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[codecov]

Codecov Report

Merging #83 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage   89.03%   89.13%   +0.09%     
==========================================
  Files          29       29              
  Lines        1423     1436      +13     
==========================================
+ Hits         1267     1280      +13     
  Misses        156      156

Impacted Files	Coverage Δ
abagen/datasets/__init__.py	100% <ø> (ø)	⬆️
abagen/datasets/datasets.py	93.18% <100%> (+1.75%)	⬆️
abagen/tests/test_datasets.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0b073f9...a47aac5. Read the comment docs.

[codecov]

Codecov Report

Merging #83 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage   89.03%   89.13%   +0.09%     
==========================================
  Files          29       29              
  Lines        1423     1436      +13     
==========================================
+ Hits         1267     1280      +13     
  Misses        156      156

Impacted Files	Coverage Δ
abagen/datasets/__init__.py	100% <ø> (ø)	⬆️
abagen/datasets/datasets.py	93.18% <100%> (+1.75%)	⬆️
abagen/tests/test_datasets.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0b073f9...a47aac5. Read the comment docs.