FIX: Handle scalar case in tests

[iraedeus]

There is some issues in mathematical functions in distributions and mixture with scalar and non-scalar inputs. For example in core/mixture.py on lines 234-253:

    def pdf(self, X: ArrayLike) -> NDArray[float64]:
        """Probability Density Function of the mixture.

        The PDF is computed as the weighted sum of the PDFs of its
        components.

        Parameters
        ----------
        X : ArrayLike
            The input data points at which to evaluate the PDF.

        Returns
        -------
        NDArray[np.float64]
            The PDF values corresponding to each point in :attr:`X`.
        """

        X = np.asarray(X, dtype=float64)
        component_pdfs = np.array([comp.pdf(X) for comp in self.components])
        return np.asarray(np.dot(self.weights, component_pdfs))

Make sure that all functions returns scalar if the input is scalar and return ndarray with internal dtype if the input is non-scalar.
Add tests to it.