adjust docstrings and type definitions for fit methods

[DennisKleinhans]

To fit the doc style of sklearn:

• removed the type definitions (np.ndarray) for X and y in the fit methods of the HLMs
• added the same type hints for X and y as in sklearn:
  • X: array-like or sparse matrix of shape (n_samples, n_features)
  • y: array-like of shape (n_samples,)

[David-Kreplin]

Can't tell what we decided:

X (array-like, sparse matrix): xxx

or

X:  array-like of shape (n_samples, n_features) ...