Mixing self-consistent quantities

Mixing various quantities in self-consistent manners are quite frequent. This module enables a variety of methods based on the Pulay (DIIS) mixing methods and may be used for externally driven SC cycles.

Container classes

Mixing makes use of so called metrics and several steps of quantities stored in history.

The basic classes that are used internally are

`History`	A history class for retaining a set of history elements
`BaseMixer`	Base class mixer
`BaseWeightMixer`	Base class mixer
`BaseHistoryWeightMixer`	Base class mixer with history
`StepMixer`	Step between different mixers in a user-defined fashion

Mixing algorithms

`LinearMixer`	Linear mixing
`AndersonMixer`	Anderson mixing
`DIISMixer`	Direct inversion of the iterative subspace (DIIS mixing)
`PulayMixer`
`AdaptiveDIISMixer`	Adapt the mixing weight according to the Lagrange multiplier
`AdaptivePulayMixer`