Moved copy_graph_to_string.c and copy_string_to_graph.c to common because they are equal for all the platforms.

Initial commit graph-copy files from iTasks-SDK repository at commit 71b3c92c (https://gitlab.science.ru.nl/clean-and-itasks/iTasks-SDK/tree/71b3c92ca46ab50ed4131edddbd9318be34e79fb/Dependencies/graph_copy).