The typing of KPoint{D} needs to be changed

[brainandforce]

Currently, KPoint{D} is a subtype of DenseVector{Float64}. In general, we should not be subtyping DenseArray at all, and because the implementation of KPoint{D} is a wrapper for a StaticArray{D,Float64} plus a Float64 shift parameter, we should define the following:

struct KPoint{D,T} <: StaticVector{D,T}
    point::SVector{D,T}
    weight::T
end

This would be a breaking change, as it would propagate to KPointMesh{D}, ReciprocalDataGrid{D}, and PlanewaveWavefunction{D}.

[brainandforce]

I have a better idea: a ShiftVector{D,R<:BySpace{D},T} <: StaticVector{D,T} type which can accomodate data shifts in either real or reciprocal space, and KPoint{D,T} aliases ShiftVector{D,ByReciprocalSpace{D},T}.

This should also dramatically simplify the new data grid implementation coming in 0.2.0.

[brainandforce]

This is already implemented in the next branch and will be resolved by #208.