Feature/irregular_structure_and_operations

[opintosant]

Full functionality of FDataIrregular structure and operations. Includes:

• Base structure. Create FDataIrregular. Load from FDataGrid or pandas.DataFrame.
• Representation: Plot and Scatter.
• Dataset 'bone_density_ext' added to _real_datasets.
• Arithmetic and numerical operations with scalars, vectors, and other FDataIrregular objects.
• Conversion from FDataIrregular to FDataGrid and FDataBasis.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.28632% with 119 lines in your changes are missing coverage. Please review.

Project coverage is 86.65%. Comparing base (adb80fe) to head (21f7bad).

Files	Patch %	Lines
skfda/representation/irregular.py	82.64%	76 Missing :warning:
skfda/exploratory/visualization/representation.py	28.30%	38 Missing :warning:
skfda/representation/basis/_fdatabasis.py	0.00%	2 Missing :warning:
skfda/tests/test_irregular_operations.py	99.25%	2 Missing :warning:
skfda/typing/_numpy.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #536 +/- ## =========================================== + Coverage 86.61% 86.65% +0.04% =========================================== Files 153 156 +3 Lines 12400 13314 +914 =========================================== + Hits 10740 11537 +797 - Misses 1660 1777 +117 ```

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