Generalize Model interface to actually behave like a table

[bgroenks96]

It's kind of weird that AbstractModel implements a Table interface but the Array interface (size, length, getindex, setindex, etc) behaves a like a vector of Params.

It would be really useful for AbstractModel to live up to its aspirations to be a real Table and allow itself to be indexed as such.

This has the potential to address #21 and #34 since it would allow for things like:

m = Model(obj)
m[.!m[:isconstant], :val] .= values

i.e. permitting both row and column indexing.

I will take a crack at it and submit a PR since this would solve a problem I am facing at the moment.

[bgroenks96]

@rafaqz do we need setindex! for AbstractModel to be allocation-free and type stable?

[rafaqz]

I dont know, but probably it cant be?

Can we use setindex for StaticModel as the type stable option?

[bgroenks96]

@rafaqz Do you have any ongoing work on this? Or should I take a crack at updating #44?

[rafaqz]

I had a play a while ago but mostly just in my head at the moment.

One thing I was thinking is we want to be able to set values by both row and column

[bgroenks96]

I think #44 did that but somewhat inefficiently. IIRC my general takeaway from working on it was that it wasn't worth it to try to make setting values type stable for the full table interface; only for updating the values. I think that's reasonable since this is more of a UI feature anyway?

[rafaqz]

We probably just need to be clearer about what features are UX improvements and which are for hot loops