Add function to add a CreateAction to the crate

[elichad]

e.g. add_action() which takes the inputs, outputs, instrument, etc as arguments and builds a CreateAction entity.

CreateAction entities are fiddly to create, but essential for WRROC - so many workflow managers need to create them and have written the code themselves (e.g. see #148). It would be useful to offer a built-in function that can help with wiring all the entities together, to make adoption easier for more workflow managers.