Taal: Use resize!able storage in trees/containers

[ranocha]

Copying a conversation from Slack:

@sloede

What Tamas proposed in https://discourse.julialang.org/t/resize-and-multidimensional-arrays/47842/4 is the way the Tree already uses right now:

If you need to resize a lot, just allocate a “large enough” array and keep track of the dimensions you use at the momene. You can wrap this in a struct and implement the AbstractArray interface very simply.

Do you think this could be an option for us as well? After all, this is what many old-school Fortran/C/C++ simulation codes that use AMR do as well

In some sense, that's what we are currently doing. resize! et al. allocate more memory than currently required to make growing faster for future calls, cf. JuliaLang/julia#31855. The relevant code is https://github.com/JuliaLang/julia/blob/master/src/array.c#L897-L905.

Would it be an option to switch to Vectors as basic storage, which can be resize!ed if necessary, and use unsafe_wrapped Arrays to interface with them?
This would remove the need to provide a capacity that's big enough. We should then implement resize! (and maybe sizehint! etc.) for these containers/trees.

@sloede:

Yes, why not? Although I'd not switch right now unless there's a compelling reason but rather finish the current Taal porting first

So we should look into this once #200 is merged into master.

Things to change

• 1D containers
• 2D containers
• 3D containers
• tree

[ranocha]

Xref #161