Implement partial_derivative_scalar in Bivariate Base class

[csala]

The method _partial_derivative is being used in the Vine classes:

https://github.com/DAI-Lab/Copulas/blob/57e4eb3a462e0ccffc25cc4bedd5a413304fe27a/copulas/multivariate/vine.py#L187

However, this method should is not intended to be called from outside the Bivariate copula classes (as it starts with an underscore), and is not implemented in all the Bivariate subclasses.

In order to fix this, the method should be moved to the Bivariate class and renamed to partial_derivative_scalar or similar.