Add PER

[maliesa96]

Closes #1489.

[maliesa96]

Benchmarks should be done in a few days.

[codecov]

Codecov Report

Merging #2159 (8f3fb8e) into master (e1c576f) will decrease coverage by 0.01%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #2159      +/-   ##
==========================================
- Coverage   91.54%   91.52%   -0.02%     
==========================================
  Files         197      198       +1     
  Lines       10927    10979      +52     
  Branches     1387     1393       +6     
==========================================
+ Hits        10003    10049      +46     
- Misses        665      669       +4     
- Partials      259      261       +2     

Impacted Files	Coverage Δ
src/garage/envs/gym_env.py	96.21% <ø> (ø)
src/garage/replay_buffer/replay_buffer.py	19.17% <ø> (ø)
src/garage/torch/algos/pearl.py	48.29% <0.00%> (ø)
src/garage/torch/algos/dqn.py	89.85% <61.53%> (-3.23%)	⬇️
src/garage/replay_buffer/per_replay_buffer.py	97.61% <97.61%> (ø)
src/garage/replay_buffer/path_buffer.py	96.07% <100.00%> (+0.07%)	⬆️
src/garage/tf/algos/ddpg.py	97.02% <100.00%> (ø)
src/garage/tf/algos/dqn.py	91.52% <100.00%> (ø)
src/garage/tf/algos/td3.py	96.66% <100.00%> (ø)
src/garage/torch/algos/ddpg.py	96.69% <100.00%> (ø)
... and 3 more

---

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[ryanjulian]

How are the benchmark results?

[krzentner]

Nice. This is a pretty clean implementation.

[maliesa96]

Currently rerunning some of the benchmarks (some were interrupted), will report back once they're done.