Standardize the `loglikelihood` and `likelihood` shape broadcasting

[michaelosthege]

Working on #12 I started changing the broadcasting behavior of CalibrationModel.likelihood and CalibrationModel.loglikelihood, but it's not as simple and deserves it's very own issue & PR.

What needs to be done:

• Come up with a comprehensive list of shape_indep, shape_obs, model_ndim, scan_x combinations and precisely define the shape_expected.
• Keep in mind that shape_indep may be higher-dimensional, for example when broadcasting with a 5-dimensional tensor..
• Critically analyze: Do we even need scan_x if the broadcasting behavior is clearly defined?
• Make the necessary changes to CalibrationModel.loglikelihood and CalibrationModel.likelihood