Implement fixed composition weights provided by the user

[SanggyuChong]

Hello,

This is an attempt at implementing the feature where users are able to define the composition weights used in training the SOAP-BPNN model.

• User, if he/she desires, can specify a dictionary of dictionaries in the training section of the YAML, where the key to the sub-dictionary is the target, and sub-dictionary consists of species atomic numbers and their fixed composition weights.
• For a behavior where the use of composition weights is to be turned off, user can use this feature and set all weights of all species to 0.
• Checks if values are provided for all species present in the training dataset(s).

Resolves #108

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📚 Documentation preview 📚: https://metatensor-models--129.org.readthedocs.build/en/129/