Eager execution and deterministic behavior

Major Features and Improvements

• Python 3.8 support
• Allow eager execution by setting with tf.config.run_functions_eagerly(True)
  or the environment variable "PHASESPACE_EAGER"
• Deterministic random number generation via seed
  or tf.random.Generator instance

Behavioral changes

Bug fixes and small changes

Requirement changes

• tighten TensorFlow to 2.3/2.4
• tighten TensorFlow Probability to 0.11/0.12

Thanks

• Remco de Boer and Stefan Pflüger for discussions on random number genration