tight_layout() compatibility

[johnomotani]

This PR attempts to make the animation controls (time-slider and play/pause button) compatible with use of tight_layout().

Previously, calling tight_layout could arrange the animations in a figure, but would not move or take into account the controls, which would end up overlapping the lower part of the animations.

This PR seems to work, but requires accessing a private variable of matplotlib.figure.Figure to get the gridspec, in order to modify it to add the controls. Also modifies a private member of matplotlib.gridspec.Gridspec to modify the number of rows. It would be nice if matplotlib provided public methods to access and update gridspecs, but as far as I can see they do not at the moment.

One issue is that the current implementation seems to reserve too much whitespace around the controls. It would be nice to find a way to allow tight_layout() to modify the amount of space above/below the slider.

[johnomotani]

The vertical spacing around the controls is set when the sub-grids are created here

animatplot/animatplot/animation.py

Lines 109 to 111 in df92ada

	button_gridspec = button_subplotspec.subgridspec(
	3, 3, width_ratios=[0.45, 0.45, 0.1],
	height_ratios=[.05, .5, .45], wspace=0., hspace=0.)

and here

animatplot/animatplot/animation.py

Lines 167 to 169 in df92ada

	slider_gridspec = slider_subplotspec.subgridspec(
	3, 1, height_ratios=[.2, .2, .6], wspace=0.,
	hspace=0.)

I guess the problem is that tight_layout cannot change these numbers, but I don't know how to create the slider or button axes in a different way so that it can modify their spacing and position.