Enables user-defined code in generated files

In some applications (i.e. Turbulent Channel) the user might want to read/update variables in the code-generated files as they might be time-dependent.

Any code-generation that uses generate_sweep_collection() i.e. generate_lbm_package(), by default creates a file with private member variables. This merger allows the user to define custom functions to access member variables.

I.e. for Turbulent Channel example:

force_updater = """void updateForce(double F_x) { F_x_ = F_x;}"""

generate_lbm_package(ctx, ..., additional_code=force_updater)

or

counter_get_set = [
    """double getCounter() { return counter_;}""",
    """void setCounter(double counter) { counter_ = counter;}"""
    ]

tke_welford_update = welford_assignments(field=sgs_tke, mean_field=mean_sgs_tke)

tke_welford_update_fct = function_generator(ctx , ...  )
    
generate_sweep_collection(ctx, ..., additional_code=counter_get_set)