Add DataLayouts convenience constructors

[charleskawczynski]

I've found myself churning a lot of code in the test suite, related to construction of datalayouts.

This PR adds new convenience constructors for datalayouts, meant to be used at the top-level (as they are not inferable).

They have a few advantages over the existing way we construct them. Concretely, they:

• Require less code duplication
• Infer Nf
• Use keywords for clarity, and are therefore order-agnostic

So, instead of writing

array = rand(FT, Nv, Nij, Nij, 3, Nh)
data = VIJFH{S, Nv, Nij}(array)

We can write:

data = VIJFH{S}(ArrayType{FT}, rand; Nv, Nij, Nh)
array = parent(data) # (if you still need `array`)

So, this will hopefully reduce the surface area of these constructors if we use them consistently in the test suite.

[Sbozzolo]

Thank you! Looks good to me!