Allow dynamic updating of ECAN stimulus and diffusion parameters

@sebastianruder FYI, this is related to a discussion we started previously.
I can think of different approaches, some of which I prototyped over the years, because I thought they might be better.   Many were interesting, none were compelling. (you can find one alternative atomspace in the link-grammar code-base).   For this case, which isn't performance-critical, you could just write a utility called "key_get_value()" or something like that.   You really don't need the ListLink, it does nothing useful.

There are other utilities, one called foreach_follow_link() or something like that, which deals with keys that have multiple values.   These could/should be modernized to look more like modern c++ foreach's.