Cleanup in activations (constraints and functions)

[carldlaird]

There is some design work to do with the activation functions and constraints to gain more code reuse and expand the list of supported activations.

This issue originally came from a PR comment by @jalving.

add_constraint=True is not used here

net_block and net are also not used in activation methods. we should discuss whether we need them. they might be there for consistency on the calling side.

Originally posted by @jalving in #24 (comment)