Introduce Cell types

[cortner]

This is a proposal based on the zoom call on 15 / 5 / 2024, that replaces #97 .

(1) remove bounding_box and boundary_conditions from the system and replace it with a cell object that encodes the same information.
(2) Implement a type Cell or ParaCell that contains fields cell_vectors and pbc (true / false)
(3) At the same time I would like to replace the AbstractVector with Vector in order to remove that type instability as well.

It would look something like this:

struct FlexibleSystem{D, TPART, TCELL} <: AbstractSystem{D}
    particles::Vector{TPART}
    cell::TCELL
    data::Dict{Symbol, Any}
end

struct ParaCell{D, T}
    cell_vectors::NTuple{D, SVector{3, T}}
    pbc::NTuple{D, Bool}
end

I think this will do two things:

• make the FlexibleSystem type stable
• increase flexibility

I'll start on the PR if I get tentative agreement from a few people.

[rkurchin]

I don't really have particularly strong feelings about how exactly we do things in the FastSystem and FlexibleSystem example implementations, but I presume this has a corresponding proposal for how the interface functions change?

I'm presuming based on our conversation earlier:

• get rid of these lines entirely
• change this so that it returns a vector of Bools with a length equal to D?

[cortner]

I would try to stay as close as possible to the current interface. I don't know exactly what changes are needed until I work on this.

[mfherbst]

I'm ok with this, provided that we have enough sugar that I don't have to go down to the cell datastructure when I just want to get the cell vectors out of a system. Because that's one thing I really dislike about the packages that abstract away the vectors in a cell structure is that this makes working with them quite inconvenient at times.

[cortner]

I would try to stay as close as possible to the current interface.

[jgreener64]

I am okay with this, I don't have strong feelings about how FastSystem and FlexibleSystem are implemented. As Rachel says it is more about the interface functions.