Allow dynamic updating of ECAN stimulus and diffusion parameters

@sebastianruder FYI, this is related to a discussion we started previously.
FYI, there is a utility, somewhat dorky, called "FollowLink" that does this for you.  Its dorky because its old and mis-designed.   But it allows you to say:

   FollowLink fl;
   Handle h = fl.follow_binary_link(ha, EVALUATION_LINK);

which, for you, would start with the config-atom as ha, and return the ListLink.