Move `add/remove_agent_from_space!` to the public API

[Datseris]

Following from the discussion here: https://discourse.julialang.org/t/remove-agent-from-space-but-not-scheduler/127842/10

The summary is: there are good reasons for users to be able to directly add or remove agents to a space, without removing them from existence (the ABM itself) alltogether.

To do this we need to make the add/remove_agent_from_space! public, and also:

• In its docstring document that this function is already called internally in add_agent! or remove_agent! and therefore does nto have to be manually called by the user if they are doing this operation.
• Test EXTENSIVELY in ALL SPACES whether using these functions directly without add/remove_agent! leads to any breakages.

This feature is a pre-requisitive step for #910 .

[Tortar]

I propose a little modification, we can use multiple dispatch instead, namely specialize remove_agent! for the spaces

[Datseris]

If there is the function remove_agent_from_space! you can apply it directly to the model without having to obtain the space. Which is in fact a great thing because we don't want users to be acting on the space directly anyways.

[Tortar]

yeah makes sense in general