Make derived observation spaces compatible with constructor.

[ChrisCummins]

This deprecates env.observation.add_derived_space() in favor of a derived_observation_spaces argument to the CompilerEnv constructor. This enabled derived spaces to be used in gym.make(...) invocations.

Fixes #461.

[codecov-commenter]

Codecov Report

Merging #463 (44d8602) into development (4d8e4dd) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #463      +/-   ##
===============================================
- Coverage        87.58%   87.55%   -0.04%     
===============================================
  Files              109      109              
  Lines             6113     6113              
===============================================
- Hits              5354     5352       -2     
- Misses             759      761       +2     

Impacted Files	Coverage Δ
compiler_gym/envs/compiler_env.py	89.47% <100.00%> (+0.05%)	⬆️
compiler_gym/envs/llvm/llvm_env.py	82.17% <100.00%> (-1.16%)	⬇️
compiler_gym/views/observation.py	100.00% <100.00%> (ø)
compiler_gym/envs/llvm/compute_observation.py	87.23% <0.00%> (-2.13%)	⬇️
...loop_tool/service/loop_tool_compilation_session.py	88.59% <0.00%> (-0.68%)	⬇️

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