adding critical radius formula + its Python bindings
fixing CUDA compilation issue in kappa_koehler.hpp
removing pov<>(si::metres) from kappa_koehler.hpp as it is not implemented in fake_units...
extending root-bracketing range as suggested by Ania Z. - unit test still to be added
Merge pull request #31 from slayoo/r_crit

adding critical radius formula + its Python bindings