Add Unit Tests

[HaaLeo]

Description

As swarmlib is growing it is essential to ensure the key functionality works flawless. Therefore at least the base components (of the utility module should be tested.

Proposal

• Add some first unit tests using pytest. Testing matplotlib related things is documented here.
• Testing animations is not supported out of the box (ref Animation comparison matplotlib/pytest-mpl#74). Therefore, for the scope of this issue it should be sufficient to initialize the BaseVisualizer and then check with ptest-mpl whether the background is drawn correctly