Upgrade to Tensorflow 2.x.x via auto upgrade

[QueensGambit]

This PR upgrades the SPFlow library to allow an execution with Tensorflow 2.x.x. I used the official auto upgrade script for migration:

• https://www.tensorflow.org/guide/migrate

tf_upgrade_v2 \
  --intree SPFlow/ \
  --outtree SPFlow_v2/ \
  --reportfile report.txt

The script added tf.compat.v1 at several places which may be addressed later.

TensorFlow 2.0 Upgrade Script
-----------------------------
Converted 213 files
Detected 16 issues that require attention

See report.txt for the full log.

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I also need to add

tf.compat.v1.disable_eager_execution()

to avoid the error

tf.placeholder() is not compatible with eager execution.

Please do not merge if you do not want to use Tensorflow in backward compatibility mode as a temporary solution.

Related Issues

• https://github.com/SPFlow/SPFlow/issues/70
• https://github.com/SPFlow/SPFlow/issues/81