Corrected typo in string.h
Moved operate, init, connect functions from op.c to nodes.c, weights.c,
network.c where they belong. They had been put in op.c for slight
simplifications of coding, but this caused more confusion and
inconveniences than anything. Next step is to rewrite threads.c to not
use op internals, so that we can get rid of the optype.h header. See
TODO.