Minor fix to get_element_profile

[jmmshn]

Minor fixes to the return types of get_element_profile and electrode

get_element_profile fix:
The entries returned is given as dict_keys but that's rarely what people want.

• Return a list of entries instead.
• Also return the critical composition, which is usually useful information.

electrode fixes:

• Linting on some docs
• require framework formula for the from_steps constructor.

[coveralls]

Coverage: 78.163% (-0.7%) from 78.873% when pulling 35ffd5e on jmmshn:patch_profil into 8f5a6f5 on materialsproject:master.

[janosh]

Thanks @jmmshn! 👍