Add a step function to the profiler to easily switch to another event of the same...

Add a step function to the profiler to easily switch to another event of the same category.

Before:                                     After:
PlProfiler::start("Machin", "truc");        PlProfiler::start("Machin", "truc");
do_truc();                                  do_truc();
PlProfiler::stop("Machin");
PlProfiler::start("Machin", "bidule");      PlProfiler::step("Machin", "bidule");
do_bidule();                                do_bidule();
PlProfiler::stop("Machin");                 PlProfiler::stop("Machin);

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>