Make `box.lo`, `box.hi`, `box.dx`, etc. dictionaries?

[RemiLehe]

Right now, box.lo, box.hi, etc. are list, so one needs to remember that e.g. box.lo[0] corresponds to x (in cartesian) and box.lo[2] corresponds to t, which can be quite error-prone.

Should we make them dictionaries instead (so that we would use box.lo['x'], box.lo['t'] instead)?

[MaxThevenet]

I agree, anything helping would be welcome here. We could also consider new member variables like box.x, box.t etc. I think @MKirchen had some ideas on Box and Grid (e.g. merging them), also to simplify the use, so maybe the can be done together.