Add support for interpolation from parallel config branches #11

In Hydra, interpolation currently only works if the reference value is defined in a higher-level node in the config hierarchy. However, interpolation fails when the config tries to reference a value from a parallel branch in the hierarchy (even though this should work in principle).

For example, the fp_length needed to initialize the input size of the MLP is currently added to the top-level config during runtime via the property system although it is already defined in the representation config. Scenarios like this occur occasionally and are annoying to deal with.

To fix this, we need to check whether Hydra really cannot do this, and possibly implement a custom config parser that resolves these values in advance. It may also be worthwhile to make a PR to Hydra itself.

This improvement would significantly enhance the usability of the ChemTorch config system.