Expose data_rw_flags to the user

[philipheinisch]

We should expose data_rw_flags to the user, to be able to specify if some varibales are read/write only or both. This could be extended to support different memory copy options for multiple kernels in the future.

[ranocha]

Cf.

toolkitICL/src/main.cpp

Line 236 in 07b1245

vector<cl_int> data_rw_flags(data_names.size(), 0);

[philipheinisch]

Exactly, this just sets everything to read & write. We should at least expand the 'TODO' to explain what data_rw_flags does or can do.