Update mean an sum functions

[aleexarias]

Update mean an sum functions for FData, FDataGrid, FDataIrregular and FDataBasis to correctly handle NaN values in coefficients.

Fixes #642

Describe the proposed changes

Edit the mean function from FData so that it only becomes a parameter check, leaving the checks as it is.
Add an auxiliar function in FDataGrid that works for mean, sum and var, and simply calls the relevant np.sum/nansum, mean/nanmean, var/nanvar when relevant depending on the skipna parameter, have the mean and sum function work with this auxiliar function.
Add a mean function in FDataBasis that calculates the means for the coefficients when the functions have no nan values in the coefficients, otherwise it is not considered for the calculations.
Add a mean function in FDataIrregular that calculates the mean based on the mean_counts parameter and depending on skipna or not.

• I have performed a self-review of my code
• The code conforms to the style used in this package
• The code is fully documented and typed (type-checked with Mypy)
• I have added thorough tests for the new/changed functionality