Replace imitation environments with seals

[Rocamonde]

Description

Fixes #501. (Depends on https://github.com/HumanCompatibleAI/seals/pull/58).

[AdamGleave]

Note will need to remove mentions of envs from the docs (especially once https://github.com/HumanCompatibleAI/imitation/pull/525/ is merged)

[codecov[bot]]

Codecov Report

Merging #541 (95d37ca) into master (531fa06) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
+ Coverage   97.45%   97.48%   +0.03%     
==========================================
  Files          88       83       -5     
  Lines        8401     8079     -322     
==========================================
- Hits         8187     7876     -311     
+ Misses        214      203      -11     

Impacted Files	Coverage Δ
src/imitation/algorithms/mce_irl.py	95.95% <100.00%> (ø)
tests/algorithms/test_mce_irl.py	100.00% <100.00%> (ø)

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[Rocamonde]

Tests are passing now. The errors were quite silly:

• The first was that the base POMDP was incrementing the number of steps after the done bool had been computed, so it was always running one step over the horizon.
• The second was that in the notebook examples, I was passing the same environment instance to each parallel vector environment instead of creating a new one.

I think that the seals pull (https://github.com/HumanCompatibleAI/seals/pull/58) should be good to merge, except perhaps for adding some tests. @AdamGleave do you have any suggestions as to what tests we could add? Once we merge that, we replace the version pointer on this PR and we can merge this too.