Since we now have access to the `AbstractElement` through the
`ComputationContext` we can just set the flag that this element relies
on tree counting functions directly, no need to pass this struct around.
There are some non-math functions (such as tree counting functions)
which we should allow within `calc()`s . This commit implements the
initial infrastructure for this.
We don't yet parse any of these non-math functions in
`parse_a_calculation` so there is no functional change.
