Breaking changes for release v0.11.0 - #373

…ly (#372)

…ials

* Updates to the DFT theory guide

* Add section on constrained DFT

* More cleanups

* Adapt Newton solver description to match EL equation and particle constraint DFT

Near hard walls / capped external potentials, FFT round-off can give
|n2v| > n2 (xi < 0) and n0 < 0. Clamp xi to [0, 1] and n0 to >= 0
before evaluating the association Helmholtz energy, which requires
rho0 >= 0.