`PredTask` add `atom_fea` and `crystal_fea`

[janosh]

I think it might be cleaner to add return_atom_feas and return_crystal_feas to the PredTask type

https://github.com/CederGroupHub/chgnet/blob/84b7c30b96b305f34abfbd8e966bac717e9723ae/chgnet/__init__.py#L15

i.e.

PRED_TASK = frozenset(["e", "f", "s", "m", "atom_fea", "crystal_fea"])

This is a breaking change but I think it would make the code cleaner (see number of added/deleted lines = 51/174).

[codacy-production[bot]]

Coverage summary from Codacy

Merging #54 (59d04e044e9d196968f833a055bddad729be2c0c) into main (a42cb35d3f4ac207c215065b96f05bf80a57bd6a) - See PR on Codacy

Coverage variation	Diff coverage
Report missing for a42cb35d3f4ac207c215065b96f05bf80a57bd6a[^1]	87.50%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (a42cb35d3f4ac207c215065b96f05bf80a57bd6a) | Report Missing | Report Missing | Report Missing | | | Head commit (59d04e044e9d196968f833a055bddad729be2c0c) | 1182 | 894 | 75.64% | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#54) | 16 | 14 | **87.50%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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