Simplify code

[GiggleLiu]

Notes

1. Avoid using abstract type as type fields

struct Lattice{T}
    vectors::AbstractMatrix{T}
    pbc::AbstractVector{Bool}
end

Should be changed to

struct Lattice{T}
    vectors::Matrix{T}
    pbc::Vector{Bool}
end

or

struct Lattice{T, MT<:AbstractMatrix{T}, VT<:AbstractVector{Bool}}
    vectors::MT
    pbc::VT
end

Otherwise, the type is unstable. For more, please check the performance tips: https://docs.julialang.org/en/v1/manual/performance-tips/

2. Symbolic type and string type.

struct Ion{T}
    species::String
    charge::Int
    frac_pos::AbstractVector{T}
end

Here, I would chose to use Symbol type rather than the String type. Symbol is faster and is more static, while having less freedom.

3. Use CartesianIndices

CartesianIndices can make you manipulate high dimensional arrays much easier. Please read this blog post:
https://julialang.org/blog/2016/02/iteration/

[Br0kenSmi1e]

Appreciate your advice!