Bug: sliders for size and #agents are there but do not work

Problem @mlerik

I've co-opted this issue for various editor issues. Not all are resolved:

(done) The sliders take effect when the regenerate button is pressed. Moved the sliders into the "Regen" tab to make this clear. (Tooltips don't seem to work on ipywidgets IntSlider)

(done) make an "Empty" generator the default for the editor.

(done) Gaps in the rails drawn apparently due to gaps in the trail of events?

• fixed by performing "manhattan interpolation" between points more than 1 cell apart.

(done) Scaling the env up to 100x100. Chrome already rescales the image a bit. It can also be shrunk using chrome zoom. I have tested it a bit with 100x100 and it's working OK for me (chrome, linux, 4.2Ghz i7, 1920x1200 and 4k screens)

(done) The trail of dots when drawing a rail only appear in the 10x10 top-left when drawing on a larger env. Needed to update the self.yxSize variable from wImage.data.shape.

(not done) Improve performance of 100x100 drawing by recording and only redrawing the dirty cells?

(not done) Rails are only drawn on a mouse-move event after the mouse drag; this leads to a strange delay when you stop the drag and keep the mouse stationary.

(not done) There are gaps in the trail of dots -

• from redrawing the rails even though no changes have yet been made.
• possibly because all the dots in the queue are drawn at once - can this be reduced by only processing say 5 dots at a time?

(not done) Various bugs with editing agent locations, clicking step, etc.

Will try to add some tests on the API between the editor and env.