Export `add_agent_to_model!` and `remove_agent_from_model!`

[Tortar]

This would create a way to do what is described in #719 and #720. Also we should export not the current functions which accept an agent, but new versions similar to the non deprecated versions of add_agent!.

Then we can add an example describing this use case in the docstring of the methods.

[Tortar]

not planned