RKHS inner product

[m5signorini]

This PR adds the RKHS inner product.

Cases regarding the product between FDataBasis objects with different basis or with the covariance function not already expressed in the tensor basis, might be better to not consider them.

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (7488932) 86.67% compared to head (5c30710) 86.14%. Report is 1 commits behind head on develop.

:exclamation: Current head 5c30710 differs from pull request most recent head 5655385. Consider uploading reports for the commit 5655385 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #550 +/- ## =========================================== - Coverage 86.67% 86.14% -0.54% =========================================== Files 153 149 -4 Lines 12390 11783 -607 =========================================== - Hits 10739 10150 -589 + Misses 1651 1633 -18 ``` | [Files](https://app.codecov.io/gh/GAA-UAM/scikit-fda/pull/550?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM) | Coverage Δ | | |---|---|---| | [skfda/misc/covariances.py](https://app.codecov.io/gh/GAA-UAM/scikit-fda/pull/550?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM#diff-c2tmZGEvbWlzYy9jb3ZhcmlhbmNlcy5weQ==) | `84.54% <100.00%> (+0.14%)` | :arrow_up: | | [skfda/tests/test\_rkhs\_inner\_product.py](https://app.codecov.io/gh/GAA-UAM/scikit-fda/pull/550?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM#diff-c2tmZGEvdGVzdHMvdGVzdF9ya2hzX2lubmVyX3Byb2R1Y3QucHk=) | `100.00% <100.00%> (ø)` | | | [skfda/misc/rkhs\_product.py](https://app.codecov.io/gh/GAA-UAM/scikit-fda/pull/550?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM#diff-c2tmZGEvbWlzYy9ya2hzX3Byb2R1Y3QucHk=) | `89.28% <89.28%> (ø)` | | ... and [37 files with indirect coverage changes](https://app.codecov.io/gh/GAA-UAM/scikit-fda/pull/550/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM)

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[m5signorini]

Current test errors occur in test_pandas_fdatabasis and test_pandas_fdatagrid . They say that the fixture all_boolean_reductions can not found.

[vnmabus]

@m5signorini Maybe I wasn't clear. I was asking if there is anything else that you wanted to include, as the PR is marked as a draft, or if I have your permission to merge it.

[m5signorini]

Yes sorry, forgot to change it from draft. I think it can be merged.

[vnmabus]

Thank you for your answer! However, a test is failing (and the difference between the expected and actual result is too high). Can you take a look at it?