Feature request: support for blended electrodes

[ejfdickinson]

Provide support in the BPX standard for electrodes to contain a specified blend of different materials (e.g. by weight or atom fraction) with discretely defined OCP functions and maximum concentrations.

[request from industrial contributor]

[valentinsulzer]

Everything currently in the "Electrode" class (as opposed to "Contact") applies to the material only. So a blended electrode would replace

negative_electrode: Electrode = Field(
        alias="Negative electrode",
    )

with a list of the materials that make up the electrode

[rtimms]

done in #33