## CustomCodeNode should be treated like SympyAssignment in transformations (currently RNG numbers are calculated too often)

I don't know if it is a bug or if I am using it wrong, but when using mutliple rng symbols via `next`

on e.g. fluxes in different directions, the generated code will calculate each random number when only one is used. so basically for a D3Q19 stencil Philox is called 5 times more then is needed, which slows down the program drastically.

```
import pystencils as ps
import sympy as sp
from pystencils.rng import random_symbol
# Setup
stencil = 19
L=(10,10,10)
dh = ps.create_data_handling(domain_size=L, periodicity=True)
c_field = dh.add_array('c', values_per_cell=1)
j_field = dh.add_array('j', values_per_cell=stencil // 2,
field_type=ps.FieldType.STAGGERED_FLUX)
def grad(f):
return sp.Matrix([ps.fd.diff(f, i) for i in range(dh.dim)])
# genergate flux eq. and set them to 0
fvm_eq = ps.fd.FVM1stOrder(c_field, flux=grad(c_field))
flux=[]
for diff in fvm_eq.discrete_flux(j_field):
flux.append(ps.Assignment(diff.lhs, 0))
flux = ps.AssignmentCollection(flux)
# define rng
rng_symbol_gen = random_symbol(flux.subexpressions, dim=dh.dim)
for i in range(len(flux.main_assignments)):
fluct = next(rng_symbol_gen)
flux.main_assignments[i] = ps.Assignment(flux.main_assignments[i].lhs, fluct)
print(flux)
flux_kernel = ps.create_staggered_kernel(flux).compile()
ps.show_code(flux_kernel)
```