PERF: Improve startup time by 0000012:0000025% delaying import of SimpleITK

Startup time reduced from 5.1s to 3.8s (cold cache)

Stats obtained after averaging values obtained after 5 runs clearing
the file cache between runs. The following command was used:

~/Projects/Slicer/Applications/SlicerApp/Testing/Python/MeasureStartupTimes.py --drop-cache -n5 --normal ./Slicer

With a warm cache, it improve the startup time by 0000021:0000032% (4s -> 2.7s)

Results have been gathered on Ubuntu 15.10 on a workstation with the
following specs: 64GB / M.2 PCIe NVMe SSD / Quad Core 3.80GHz

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25138 3bd1e089-480b-0410-8dfb-8563597acbee