Polish fpca regression example

[Ddelval]

Several issues have been addressed:

• Fix the labels of the cross validation plot
• Change cross validation plot style
• Round scores to four decimal places

[Ddelval]

Blocked until tests are fixed ( #553 )

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (680fc1e) 85.86% compared to head (9c19d28) 85.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #552 +/- ## ======================================== Coverage 85.86% 85.86% ======================================== Files 147 147 Lines 11649 11649 ======================================== Hits 10002 10002 Misses 1647 1647 ```

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