Projection input/output function refactor

[jlstevens]

Projection input and output functions were not being handled properly; state_push and state_pop was not being called for any of these transfer functions. This PR implements two changes:

• Moves the definition of the input_fns parameter to the Projection level so it can be defined in the same place as output_fns.
• Implements state_push and state_pop properly at the Projection level to correctly handle both types of transfer function.

All the training tests pass after making these modification and I don't foresee any other issues e.g snapshots should work fine after this change. I've submitted this as a PR to make sure everyone is happy with these commits before touching components in topo.base.

EDIT: As only a few lines have changed, I am happy to make these commits directly to master instead to avoid creating an additional PR commit.

[philippjfr]

Looks good to me. Shouldn't affect anything since no model that I'm aware of is actually using projection input/output functions.

[jbednar]

Looks good; merge it!