Create a FeatureOperations class that handles FeatureOperation instances

[lorenz-gorini]

Methods in DataFrameWithInfo class like add_operation or find_operation_in_column are not fully related to DataFrameWithInfo class. It could be useful to gather them inside a new FeatureOperations class that includes methods that handle FeatureOperation instances, able to add a new operation to the FeatureOperations instance, or finding an operation. Therefore the feature_elaborations attribute of DataFrameWithInfo class could be an instance of the newly created FeatureOperations class.

[alessiamarcolini]

Given the new structure of feature_operations module, this is not relevant anymore