kernel.split_particle returns by reference in Numba

[ilhamv]

The use of kernel.split_particle is treated as a call by reference in Numba. The current use of the function is always followed by kernel.add_particle, which passes by value to particle banks. However, if we (naively) use it to create intermediate particles for some operations (like in the embedded sensitivity method), this would cause issues. What is interesting is that the function returns by value in Python.