update io __init__.py with attempt import

[jalving]

This PR uses the pyomo attempt_import to defer the import of dependent packages until they are used (or to check if the package exists). See src/omlt/dependencies.py. This should address issue #96.

The function attempt_import offers a lot of extra functionality - not sure whether we need to use all of it.

E.g., to check if keras (or onnx) is available, use: from omlt.dependencies import keras_available

E.g., to import keras (or onnx) use: from omlt.dependencies import keras

If you then try to use keras and it is not installed, Pyomo's attempt import will throw an exception.

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[codecov[bot]]

Codecov Report

Merging #97 (0f55ad2) into main (d688364) will decrease coverage by 0.11%. The diff coverage is 77.77%.

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
- Coverage   95.08%   94.97%   -0.12%     
==========================================
  Files          25       26       +1     
  Lines        1364     1372       +8     
  Branches      189      191       +2     
==========================================
+ Hits         1297     1303       +6     
  Misses         37       37              
- Partials       30       32       +2     

Impacted Files	Coverage Δ
src/omlt/io/__init__.py	60.00% <60.00%> (-40.00%)	:arrow_down:
src/omlt/dependencies.py	100.00% <100.00%> (ø)
src/omlt/formulation.py	92.64% <100.00%> (+0.10%)	:arrow_up:

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[jalving]

a couple quick notes:

• flake8 was erroring because @abstractmethod was not defined on the __init__ method in _PyomoFormulationInterface. I am testing with the same flake8 version locally which did not error. I have not found why it happens on Github.
• the patch coverage fails because we don't test attempt_import for the case where we don't have tensorflow or onnx. Those tests might be more cumbersome to write. @carldlaird do you have any ideas?