This commit introduce coverage via 3 elements under the control of new
USE_LCOV option:
 * compilation/linking option (note that it is specific to GCC,
   typically on liger with intel compiler, option is not recognized).
 * add a target lcov that create a coverage folder in
   CMAKE_BINARY_DIR  and collect in this folder all run results
   present in all directories and sub directories under
   CMAKE_BINARY_DIR and then create a html interpretation of those
   results in coverage folder.
 * add a target cleanlcov that removes coverage folder and all run
   results present under CMAKE_BINARY_DIR.

To do some coverage turn on USE_LCOV option, recompile all or parts of
your project, run all or part of your project executable and then type
make lcov.
You will obtain a coverage view of your project for part compiled with
coverage and run.
You may run new part of your project and type make lcov to obtain
merging coverage view. But if you want to see only coverage view of a
specific program, type first make cleanlcov, then run your specific
program and finally type again make lcov.

Use any web browser with coverage/index.html to see coverage view
generate by make lcov

Note: It is a quick preliminary implantation and may deserve more
attention to fill our needs
lcov is not installed on titan for now so LocalConfig_Titan is turning
off USE_LCOV option.
On liger lcov is installed but do not work with intel compiler.