Inconsistent semantics of 0-dim `RArray`s compared to Julia semantics

[mofeing]

#51 fixed the semantics problem for all N-dim RArrays except for 0-dim RArrays, which we treat as scalars. Although mathematically accurate, there is a distinction between a Number and an AbstractArray{T,0} in Julia which we should respect to ensure compatibility between both.

One proposal is to add a new type for traced scalars... My doubt is to whether TracedRNumber{T} <: Number would be enough or do we need sth like TracedRFloat{F<:AbstractFloat} <: AbstractFloat for each number type we are interested in.

CC @avik-pal