No default rounding to `center` method

Working through #242 #251 and #254, I have to admit that I made a design mistake here, instead of current

function center(A::AbstractArray, r::RoundingMode=RoundDown)
    map(axes(A)) do inds
        round(Int, (length(inds)-1)/2, r) + first(inds)
    end
end
centered(A::AbstractArray, cp::Dims=center(A)) = OffsetArray(A, .-cp)

A better approach is perhaps to:

function center(A::AbstractArray)
    map(axes(A)) do inds
        length(inds)-1)/2 + first(inds)
    end
end
function centered(A::AbstractArray, cp::Dims=center(A), r::RoundingMode=RoundDown)
    OffsetArray(A, map(x->round(Int, x, r), .-cp))
end

This enables users to get the raw float point center coordinate and thus can be used in other scenarios. For instance, in interpolations, arrays are not required to be placed at the grids points. Also, unlike the previous version, applying rounding mode to centered will not be ambiguous anymore (See #250)

Unfortunately, to support this breaking behavior transition, a major version bump is required. For now, we should just stick to the current integer world. Also, I'm not sure how much people want this change so it's better to leave it open for a while before making any deprecations.

Read the current center and centered definitions, then review the discussions in #242, #251, #254, and #250. Determine whether the proposed separation of raw center coordinates from rounding in centered is accepted; done would require a maintainer decision on the breaking-change and versioning plan, since this issue currently leaves the change open.