Indicate output dimensionality in `AbstractInterpolation` type

[adrhill]

As a developer writing a package extension for DataInterpolations.jl (adrhill/SparseConnectivityTracer.jl#178), it would be useful if all AbstractInterpolations indicated their output dimensionality, e.g.

abstract type AbstractInterpolation{T,N} end 

or {N,T} instead of just {T}:

DataInterpolations.jl/src/DataInterpolations.jl

Line 5 in 536cb02

abstract type AbstractInterpolation{T} end

[ChrisRackauckas]

That's probably fine.

[sathvikbhagavan]

Yeah, that can be done. Will work on it soon.