PlotDataItem downsampling calculation fix

Added check for zero ViewBox width when calculating automatic downsampling ratio. (Prevents zero-division when downsampling is set before Plot is properly created and drawn within a container).

Added several utilities and debugging tools
Merge branch 'multiprocess-fix' into develop

Multiprocess debugging messages now use one color per process
Corrected RemoteGraphicsView not setting correct pg options on remote
process

New debugging tools:
* util.cprint for printing color on terminal (based on colorama)
* debug.ThreadColor causes each thread to print in a different color
* debug.PeriodicTrace used for debugging deadlocks
* Mutex for detecting deadlocks

Merge branch 'make_scipy_optional' into develop

Several examples still require scipy.

Merge remote-tracking branch 'zarch/bottleneck_range' into develop

Update MultiPlotSpeedTest to test lru_cache performance

Changed FiniteCache which had a bug where calling items() would make it recurse forever with a new LRUCache implementation.

Fixed crash when collecting items on ViewBox and fixed exception which could happen when dragging when mouse handling is disabled.

Merge remote-tracking branch 'termim/develop' into develop

Conflicts:
	pyqtgraph/graphicsItems/ROI.py