Allow BasisSmoother to accept irregularly sampled data

[vnmabus]

Is your feature request related to a problem? Please describe.
Currently, scikit-fda does not have any type for irregularly sampled data. A fully-fledged irregular type will require weeks or months of design and coding. However, if BasisSmoother could accept a list of FDataGrid as input, irregular data could be converted to a basis form as part of a pipeline.

Describe the solution you'd like
Implement BasisSmoother for lists of FDataGrid, at least until we have a proper type for irregular data.

Describe alternatives you've considered
A utility function could be provided instead, but it could not be used in a pipeline, which is important to adjust the hyperparameters.

Additional context
Irregularly sampled data arises in different contexts, such as in medicine. A way to convert this to a basis expansion in order to use FDA functionality would be useful for these use cases.

[vnmabus]

Some of the methods of BasisSmoother, such as the method to compute the hat matrix, make no sense in the sparse context. For now we can ignore it, but we will need to generalize that in the future.