Remove maggma dependence, add conversion electrode rester

[esoteric-ephemera]

Can be merged once emmet-core #1267 is merged/released.

• Migrates sole maggma import to emmet.core, removes maggma from dependencies.
• Adds ConversionElectrodeRester to query the conversion_electrodes endpoint

[esoteric-ephemera]

@tschaume there seem to be some issues with the conversion_electrodes endpoint, perhaps that's why a client search feature wasn't added for it before? Have basically flagged these in the class itself under _exclude_search_fields

• material_ids is listed as a valid search term in the docs, but there are neither material_id or material_ids fields in the schema
• formula, chemsys, and elements do not return any matches despite manually checking values from OpenData
• stability_charge and stability_discharge are also not working
• Similarly, passing exclude_elements will return the entire collection

[tschaume]

Looks good!

The issues you found are likely due to the the query operators defined here. The multi-material query isn't needed, for instance. It could be that it was a simple copy-paste oversight when the endpoint was added in emmet PR 814.

@kbuma maybe you could take a closer look at the operator definitions for the conversion electrodes endpoints as part of your cleanup in emmet PR 1282? Thanks!

[tschaume]

remove