Add missing data imputation via interpolation

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 :tada:

Comparison is base (4865912) 85.71% compared to head (5760d9f) 85.75%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #533 +/- ## =========================================== + Coverage 85.71% 85.75% +0.03% =========================================== Files 143 144 +1 Lines 11396 11428 +32 =========================================== + Hits 9768 9800 +32 Misses 1628 1628 ``` | [Impacted Files](https://codecov.io/gh/GAA-UAM/scikit-fda/pull/533?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM) | Coverage Δ | | |---|---|---| | [skfda/preprocessing/missing/\_interpolate.py](https://codecov.io/gh/GAA-UAM/scikit-fda/pull/533?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM#diff-c2tmZGEvcHJlcHJvY2Vzc2luZy9taXNzaW5nL19pbnRlcnBvbGF0ZS5weQ==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM)

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