New base class `KineticBackground`

Solves the following issue(s):

Closes #276 (closed)

Core changes:

• Added new base class KineticBackground with the magic methods __add__, __mul__, __rmul__ and __sub__. The Maxwellians now inherit this base class, class Maxwellian(KineticBackground). A unit test for the magic methods was added.

• Added @abstractmethods n for density and u for mean velocity to kinetic background. Enabled lists for background in parameter file.

• In the parameter file, this enables to specify multiple backgrounds of the same type by appending _1, _2 etc. to the type name. This is explained in the updated doc under userguide/initial_conditions.

• This enables simulations of with two-stream and bump-on-tail equilibria for instance. This can be tested with the file two_stream.yml.

Model-specific changes:

None

Documentation changes:

Userguide/initial_conditions