Allow for the refresh rate to be specified

[philipbelesky]

Hey, thanks for all the recent updates! I'm not myself sold on whether this is a good idea (and am mindful of settings overload) but there were a couple of times when I wanted to be able to control the interval between the solution expiring. These were:

• When I first started trying out the new version I had disabled the discrete GPU (long story; trying to get an external GPU running) and as a result found the performance of GH very slow. The default 20ms interval would (somewhat) slowly update in Rhino, but the definition itself was basically un-interactive. With the main GPU running again everything was fine. But it may be worth allowing users to specify a longer interval if they are running with truly bad hardware
• Related to the above: adding downstream components that do relatively heavy computations seems to essentially add latency to interacting with the definition
• When developing a definition it can be nice to have a 'manual update' mode where the mesh only updates on a forced recalculation of the definition (the slight jumping/shifting of the geometry can be distracting)

[mariuszhermansdorfer]

The more robust approach would be to spawn the computation on a separate thread, so that it doesn't lock the UI thread. Opened #18