Improve Environment Management

[dlshriver]

Refactors the environment management system to be written in python. This simplifies verifier installation, and enables verifiers to be installed when DNNV has been installed using pip. The docs are updated to reflect these changes.

[codecov-commenter]

Codecov Report

Merging #44 (b3cc9f2) into develop (fe9d547) will decrease coverage by 4.56%.
The diff coverage is 17.72%.

@@             Coverage Diff             @@
##           develop      #44      +/-   ##
===========================================
- Coverage    63.60%   59.04%   -4.57%     
===========================================
  Files           88      109      +21     
  Lines         6556     7139     +583     
  Branches      1257     1298      +41     
===========================================
+ Hits          4170     4215      +45     
- Misses        2020     2582     +562     
+ Partials       366      342      -24     

Impacted Files	Coverage Δ
dnnv/__init__.py	100.00% <ø> (ø)
dnnv/__main__.py	0.00% <0.00%> (ø)
dnnv/_manage/__main__.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/bab.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/babsb.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/eran.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/marabou.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/mipverify.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/neurify.py	0.00% <0.00%> (ø)
dnnv/_manage/linux/verifiers/nnenum.py	0.00% <0.00%> (ø)
... and 76 more

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