Generalize Array interface to 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?