Add total_elements to dace.data descriptors, next to total_size (including Scalar)

Data descriptors expose `total_size`, but there is no `total_elements`. `Array.total_size` is the allocated span, which can exceed the logical element count when strides or padding are given, so callers that need the number of elements recompute `prod(shape)` themselves.

Proposal: add a `total_elements` property to `dace.data` (at least `Data`, `Array`, `Scalar`, `Stream`), with `Scalar` returning 1, so that `total_elements` and `total_size` are both valid on every descriptor, scalars included.

Start by locating the dace.data descriptor classes and the existing total_size implementations. Trace Data, Array, Scalar, and Stream to identify where the shared property belongs and how scalar descriptors are handled. Done means every listed descriptor exposes total_elements, Scalar returns 1, and the logical element count remains distinct from allocated total_size.

Written by the indexing model from the issue text.