recenter: accept origin::Tuple

[johnnychen94]

OffsetArrays.center is added in JuliaArrays/OffsetArrays.jl#242, and I think it makes sense to remove ImageTransformations.center in favor of OffsetArrays.center

# https://github.com/JuliaImages/ImageTransformations.jl/blob/master/src/ImageTransformations.jl#L43-L44
center(img::AbstractArray{T,N}) where {T,N} = SVector{N}(map(_center, axes(img)))
_center(ind::AbstractUnitRange) = (first(ind)+last(ind))/2

The only reason that SVector is used here is because CoordinateTransformations.recenter requires AbstractVector input, I relax this constrain in this PR so that OffsetArrays.center can be a drop-in replacement for ImageTransformations.center (if we ignore the integer rounding differences).

[johnnychen94]

bump :)

[c42f]

It seems kind of strange to allow a linear map to operate on things which aren't vectors, but I guess it's also fairly harmless.