Added static option to HTML component

[arvoelke]

This is useful if you are serving something like an iframe containing javascript, where you don't want it to update every simulation timestep (i.e. served once every time the components are created). Tested manually with the following code:

def function(t): return None
function._nengo_html_ = '<iframe src="http://localhost:%d/" />' % port
function._nengo_static_ = True
nengo.Node(function, size_in=0, label="")

[mention-bot]

By analyzing the blame information on this pull request, we identified @tcstewar and @Seanny123 to be potential reviewers

[Seanny123]

Practically, do you have an example of how you would use this usefully? Just might be useful to point people to later.

[arvoelke]

I may have something nice to point to in the very near future, but for now just imagine localhost:port is some web application that is communicating with the model through another channel (via some thread inside the model).

[Seanny123]

LGTM.

[tcstewar]

Nice. I'm fine with _html_static_ for now, but I also wonder whether another option like letting people optionally return a dictionary would work:

_html_nengo_ = dict(html='<h1>title</h1>', static=True)

[Seanny123]

@tcstewar make an issue after the merge?