Adds ONNX Surrogate support from OMLT

[avdudchenko]

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Fixes

This adds ONNX surrogate model object and slightly refactors the current Keras surrogate model to support multiple omlt model types.

Summary/Motivation:

Currently, if one generates a NN model using PyTorch, or any other method for use in IDAES and depended project (e.g. WaterTAP) we must export the model to ONNX format, and then to keras format to use in IDAES, which makes little sense.

This PR adds native support to onnx model format provided through OMLT.

Changes proposed in this PR:

-move out part of Keras functions into omlt_surrogate_base.py to enable simple support for multiple omlt import types -adds ONNXSurrogate object that can load onnx model directly in IDAES flowsheet block

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