BECs and order parameters.
BoseGas(V, g, N, μ, Ω, ψ₀, nnc)
Unit scaling? Should have ω_perp=1 for V=r².
Specification can be extended or overridden with e.g. BoseGas(system; nnc=whatever).
Constructor BoseGas(::Grid; g, N) for untrapped gas.
oprm!(system::BoseGas; numerical options)
Extend with ψ₀. Exactly one of μ or N must be specified, the other is set. Self-consistent with nnc.
Functions for Hamiltonians, angular momentum and so on. These return DiffEqOperators that act on a Field.