Transformer class for the property selection

[hurricane642]

Hello, everyone! This PR is the first step in adding the 'Transformer' class to rascaline. The 'Transformer' class makes it easy to create a representation matrix when using some other matrix as a reference. A classic use case is to create a TensorMap representation for a dataset, then perform transformations within that TensorMap (e.g., keys_to_features or keys_to_properties), and select the most useful features in the transformed TensorMap. The 'Transformer' allows a set of these features to be used to calculate a new TensorMap, thus saving computation time and maintaining a single I tried to explain the logic of the algorithm in the comments to the code, but if you have any questions - ask, I'll be happy to answer, as well as happy to receive any comments and suggestions.

IMPORTANT: We need to add to the code the ability to perform several transformations one by one. This will be the next step. Note that this branch is based on the selected_keys branch, which itself is still a PR. representation for all representations.