Add pin_ptr macro

[JR-1991]

This PR adds a macro to streamline the typical pinning process upon initialization of an SBML struct. Effectively, the process reduces the following way:

// From this
let species_ptr: &mut sbmlcxx::Species = unsafe { &mut *some_ptr }
let species = unsafe { Pin::new_unchecked(ref_ptr) }

// To this
let mut species = pin_ptr!(species_ptr, sbmlcxx::Species);

While this is a minor change it streamlines the unsafe process and possibly prevents misuse of unsafe code in other places.