Descriptor convergence

[nedtaylor]

Added

• convergence criterion for update() procedure
• graphene grain boundary example, comparable to that found in https://doi.org/10.1103/PhysRevB.90.035424 (but searching over only one stoichiometry, which is the best one identified in the paper).

Fixed

• method_ratio allowed keys void typo
• elements provided to set_element_energies was not adding them to the generator unless the element also appeared in the database