[REF] Massive reduction in memory usage

[rmarkello]

Hopefully closes #99

This is an attempt to reduce the memory overhead of get_expression_data. Still a bit slow, but i think these improvements do a lot of legwork and should get us towards where we want to be.

To Do:

• Check that CircleCI doctests works
• Remove final Travis tests if CircleCI works

[codecov]

Codecov Report

Merging #130 into master will decrease coverage by 0.19%.
The diff coverage is 94.64%.

@@            Coverage Diff            @@
##           master     #130     +/-   ##
=========================================
- Coverage   90.35%   90.15%   -0.2%     
=========================================
  Files          32       32             
  Lines        1991     1961     -30     
=========================================
- Hits         1799     1768     -31     
- Misses        192      193      +1

Impacted Files	Coverage Δ
abagen/probes.py	97.11% <100%> (+0.05%)	⬆️
abagen/tests/test_probes.py	100% <100%> (ø)	⬆️
abagen/tests/test_samples.py	100% <100%> (ø)	⬆️
abagen/allen.py	97.97% <75%> (-0.1%)	⬇️
abagen/samples.py	99.04% <96.87%> (-0.96%)	⬇️

---

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 de700b5...976ec2b. Read the comment docs.