COMP: Update SlicerExecutionModel

This commit both (1) fixes CMake 3.0 config warnings (see below) and
(2) updates the project to include fixes and improvements contributed by
the community:

//------------------------------
$ git shortlog 46f3d89..865e2a7 --no-merges
Jean-Christophe Fillion-Robin (4):
ENH: Set default value for CMAKE_BUILD_TYPE
ENH: Add support for CMake 3.0 removing use of LOCATION target property
ENH: Re-factor GenerateCLP tests to support multi config system
COMP: By default, disable JsonCpp support.

Jim Miller (2):
Update README.md
Update README.md

Julien Finet (1):
Don't search default path when JsonCpp_DIR is provided

Matt McCormick (3):
BUG: Output vector argument should not be assigned.
BUG: Serialize a geometry Parameter with aggregate="true", multiple="true".
BUG: Fix serialization/deserialization with not Multiple but vector.

Matthew Woehlke (2):
ENH: Refactor [de]serialize format for brevity
STYLE: Avoid file(GLOB)

ivmartel (1):
Allow for reference attribute for geometries.
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This commit fixes the following warning:

//--------------------
CMake Warning (dev) at GenerateCLP/GenerateGenerateCLPConfig.cmake:7 (get_target_property):
Policy CMP0026 is not set: Disallow use of the LOCATION target property.
Run "cmake --help-policy CMP0026" for policy details. Use the cmake_policy
command to set the policy and suppress this warning.

The LOCATION property should not be read from target "GenerateCLPLauncher".
Use the target name directly with add_custom_command, or use the generator
expression $<TARGET_FILE>, as appropriate.

Call Stack (most recent call first):
GenerateCLP/CMakeLists.txt:201 (include)
This warning is for project developers. Use -Wno-dev to suppress it.
//--------------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23340 3bd1e089-480b-0410-8dfb-8563597acbee