Output naming

The name and format of the outputs in metatrain are based on those of the metatomic package. An immediate example is given by the energy output.

Any additional outputs present within the library are denoted by the mtt:: prefix. For example, some models can output their last-layer features, which are named as mtt::aux::{target}_last_layer_features, where aux denotes an auxiliary output.

Outputs that are specific to a particular model should be named as mtt::<model_name>::<output_name>.