Allow central bodies which have another primary than the Sun

# Feature
## Desired Behavior / Functionality
Allowing other central bodies such as the Moon.

## What Needs to Be Done
Right now, we convert to heliocentric coordinates to compute eclipses using pykep.

This conversion needs to be specified in some way if the body's motion around the sun is not implicitly modelled via pykep.

Atm, the conversion to heliocentric happens in the code at https://github.com/aidotse/PASEOS/blob/fca6c4bce6815b0415112c8e06f8be806d7a6f78/paseos/central_body/central_body.py#L162

## How Can It Be Tested

Create a lunar scenario :v: