Test if Travis VMs stalls because they run out of memory

Cmake version 3.0.2 still doesn't support CMAKE_CXX_STANDARD 11

TODO: Test fails because of problems with setting up multiple instances
of xout in the same process

Also introduces the “SUPERELATIX_RUN_LONG_TESTS” option that disables
long running tests by default (e.g. 3D and 4D registration tests)

COMP: Add flags for Linux SuperBuild, move compiler-dependent settings to the same place in the build system

TFixedImage as the ImageSource template since the output image is
always resampled to the fixed image domain.

TODO: Fix 'error while setting up xout' error when running multiple
ElastixFilters in the same binary (see ELASTIX-56 in SuperElastix Jira)

Adds a stand-alone ITK filter that wraps the elastix library
in a fully pipeline-compatible process object. The original
library interface was deprecated in favor of direct use of
elastix internals in the filter's GenerateData() for more
control over pipeline inputs and outputs.

TODO: Support multiple fixed and moving images
TODO: Return TransformParameters

WIP: Segmentation fault when trying to access result image