Makes target visible

[S010MON]

Adds a flag to the center of the target to make it visible to agents

closes #327

[Jolijn2020]

It seems like this approach would mean that we have to create a new class for every type of flag we want to use, instead of just using a different enum Type.

I understand that instead of being able to see the whole area, you just want to use a small marker in the middle of it, but this might become a problem, or at least an inconvenience in two scenario's:

1. if the agent has a part of the Target in view, but not the middle, they can't see it.
2. If agents start using flags to communicate, they might block of walls or just open space. And since there is currently no distinction between an object being visible and being solid, agent would not be able to walk over it. And if you add that distinction, it can block the agents vision.

I'm not certain if those things are worth it, just to not have to store boolean variables.
The other approach also makes it possible to make things like glass and portals visible (debate whether it's necessary for glass, but portal would be very usefull)

[MatthewFoster02]

When I run the code, agents can move through glass.

[S010MON]

Closed in favour of #328

[Jolijn2020]

Target uses 2 different classes implementing Boundary, why are they different classes?