ENH: Add to itk_common a hook after ctest_start

Add a "dashboard_hook_started" hook after the ctest_start call.
This gives scripts a chance to override the submission information
that ctest_start loaded from the project's CTestConfig.cmake file.
ENH: Drop unused itkmodular_common script

The ITKModular dashboard has no submissions and is not monitored.
Drop the client-side script supporting it.
BUG: Update CDash URL.

Change-Id: I5ef11e6a4d6a7fef9cc240419ba44988daa8e0ef
ENH: Add documentation for allowing only a specific target to be built

CTEST_BUILD_TARGET is a CTest special variable that accepts the name of a
specific build target, to be built instead of the implied "all".

CTEST_BUILD_TARGET only works if CTEST_BUILD_COMMAND is not defined (which
it typically isn't for ITK dashboard builds).

Change-Id: I3f54f38e20e7d476b763d82be428a7e8a498c971
ENH: Allow ExternalData path to be set by environment on dashboards

The new priority for setting the value of "ExternalData_OBJECT_STORES"
that is passed to any new ITK build is:
1. A CMake variable in the client configuration, "ExternalData_OBJECT_STORES"
2. An environment variable, "ExternalData_OBJECT_STORES"
3. The path "${CTEST_DASHBOARD_ROOT}/${dashboard_data_name}"
4. The path "${CTEST_DASHBOARD_ROOT}/ExternalData"

Change-Id: Ifb2b1bf07c114efb21b18c8fca615f7589ef5715
WIP: adding optional arguments for ctest_memcheck and ctest_converage