ENH Gif duration as framerate instead of duration

[wiheto]

Currently gif duration is in ms regardless of number of figures. Since gif will generally be combined with the frames argument, a gifframerate to replace gifduration is probably better.

Or at least as a possible alternative. So that if gifframerate is not None, gifduration = (framerate * 1000) * n_frames.

I think multiply with a 1000 cause framerate is natural in seconds and gifduration is currently in ms.

[wiheto]

Above was slightly inaccurate. This is not really the case, gifduration of a frame.

What is needed:

• specify frames per second instead of duration of a single frame so gifduration = (1 / gifframerate) * 1000 is the conversion
• fix documentation.