Add method to AveragedField to prescribed averaged dimensions with names, e.g., dims = (:x, :y)

[navidcy]

Oceananigans.jl/src/Fields/averaged_field.jl

Lines 38 to 39 in 73be08d

	Returns an AveragedField averaged over `dims`. `dims` is a tuple of integers indicating
	spatial dimensions; in a Cartesian coordinate system, `1=x, `2=y`, and `3=z`.

[glwagner]

Related to #457 I think. We've discussing sub typing AbstractDimArray from https://github.com/rafaqz/DimensionalData.jlhttps://github.com/rafaqz/DimensionalData.jl

We'd still have to manually add support in AveragedField however.

[glwagner]

Let's table this, one nice thing about numbers is that we avoid the need for "x" and "lambda"