Core/Rules: Better/Exposed Stabilizer

[chr5tphr]

• exposes a stabilizer parameter for all built-in rules other than
  epsilon

• implements Stabilizer as a class

• create a stabilizer function from the stabilizer/epsilon value, for
  which Stabilizer.ensure is implemented, for both the Epsilon rule
  and all other rules that apply

• stabilizer/epsilon can be a float/int, in which case a Stabilizer
  object will be instantiated with the appropriate epsilon value

• stabilizer/epsilon can be callable, in which case the callable itself
  will be used as a stabilizer

• add batchnorm to the layer_map_base, and make layer_map_base a
  function such that stabilizer for Norm() may be passed

• added tests for Stabilizer

• added explanations for the use of the new stabilizer and epsilon
  arguments to how-to/use-rules-composites-and-canonizers.rst

• BatchNorm is now ignored by default instead of using its gradient (!!)
  the reason for this choice is that it is a better choice than using
  the gradient