Get data pairs per index method + fix directory bug

[MBaltz]

Method: _get_data_pairs_perindex

• This method receive a list of index of data pairs.
• It's like a upgrade of the _get_datapairs method (but it was not overwritten).

Example of method use:

test_data = dataset.get_data_pairs_per_index('test', [1, 5, 6, 80, 150, 235])

Directory bug

• The class StandardLearnedReconstructor was not fount, I update the import directory to correct local:

  from dival.reconstructors import StandardLearnedReconstructor

  to

  from dival.reconstructors.standard_learned_reconstructor import StandardLearnedReconstructor

[jleuschn]

Thanks for adding this :+1: