Seperate classes for `receiver` and `calibration_target`

[MarleneBusch]

Problem
Right now both the calibration targets and the receiver are modeled as a flat plane. Therefore they are simulated by the same class: TargetArea. Real receivers can have unique geometries, that are sometimes not just a flat plane.

Solution
Once we use more accurate receivers we should create seperate classes for calibration targets, that are usually flat and receivers that probably are not flat.