Deepreg: Suppress TensorFlow warnings

[mathpluscode]

Subject of the feature

TensorFlow is not printing many warnings in the bash. We need to check if any one of them should be fixed, like #428.
Then we should remove warning by default.

UPDATE: the better option is to use verbosity

[mathpluscode]

defining TF_CPP_MIN_LOG_LEVEL=3 before launching the code seems to work, but it is unclear yet how to do it inside python.

for deprecated warnings, need to do
https://www.codespeedy.com/suppression-of-deprecation-warnings-in-tensorflow-python/

[zacbaum]

As mentioned in #656; would be good to adjust verbosity of logging here as well (for model.summary and model.fit as examples)

[mathpluscode]

Closing this issue in favor of https://github.com/DeepRegNet/DeepReg/issues/745