DocString incorrect: utils.measurement.synthetic_functions SyntheticFunction

[ClaasRostock]

In class SyntheticFunction, the DocString of method _f() does not seem to be correct. It was apparently copied from method f().
However, while f() works on an input matrix and returns a vector, _f() works on a single observation, i.e. vector, and returns a float.

Proposal:
def _f(self, X: np.ndarray) -> float:
...
Args:
X (numpy.ndarray): 1-dimensional input vector with d elements, where d is the dimensionality of the inputs.
Returns:
float: the function value

[ClaasRostock]

further observation: _f() is declared as class method, but has 'self' as first parameter. I suspect the @classmethod decorator is wrong and should be removed, unless _f() is really used somewhere as class method.

[esantorella]

Closed by #2329 .