deprecate `varfinite` in favor of `varmult_finite`

[johnnychen94]

Because it is unclear what operation is applied when we compute abs2(c::RGB), in ColorVectorSpace v0.9 we've removed abs2 and deprecated var in favor of varmult. Then we reimplemented abs2 again with proper deprecation (see also JuliaGraphics/ColorVectorSpace.jl#172)

In #17 efforts are paid to provide a consistent and more functional API for meanfinite, minimum_finite, maximum_finite and sumfinite to their corresponding mean, minimum, maximum and sum. It makes sense to do the same thing for varfinite here.

The proposed API is:

varmult(op, itr; corrected::Bool=true, mean=Statistics.mean(itr), dims=:)
varmult_finite(op, itr; corrected::Bool=true, mean=Statistics.mean(itr), dims=:)

with deprecation

# ⋅ is the inner product
@deprecate var(A; kwargs...) varmult_finite(⋅, A; kwargs...)

cc: @jagoosw