SampleContainer can not save metadata correctly into h5 backend

[qchempku2017]

When saving samplercontainer in h5py mode, function _init_backed failed to run becase in container.py line 410, json.dump(self.metadata) returns error. This happens because keys of Ensemble thermo_boundaries are not serialized into string, and their keys still include Species type object.

Expected Behavior

Current Behavior

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[lbluque]

Good catch @qchempku2017 ! I'll try to fix it asap

[qchempku2017]

It's caused by lack of serialization for Species as the key to chemical potentials. One suggested fix is to serialize species into strings, then use serialized chemical potentials to write.

And we definitely need a test for h5py backend dumping functionalities in container.

[qchempku2017]

So we do have a test on hdf5 save and load, but the container was constructed ad-hoc for that particular test, and it did not have metadata. We should add a proper metadata into the container and make sure the tests are passed with metadata.