Change covariance function and smoothing kernel nomenclature

[vnmabus]

Motivation

The covariance functions in scikit-fda were originally used to generate Gaussian processes. Now, their inputs will be generalized to accept also FData objects, in order to compute a "similarity measure" between them. The name "covariance function" is not used in this context, preferring the name "kernel" instead. Currently, this name is used for the smoothing kernel, which is ALSO called kernel in the literature.

Desired functionality

Covariance functions should be renamed to "kernel", as this name is used in the literature for Gaussian processes and to compute similarities. The smoothing kernels should be renamed to SmoothingKernel, KDEKernel or something similar.

Alternatives

No response

Additional context

No response