Fix Scheduler::StaticSchedulingElement shared_ptr circular reference

Context

See aidge#195

Solve an indirect leak in Scheduler class like described in #184 (closed)

Modified files

• Scheduler.hpp/cpp
• SequentialScheduler.hpp/cpp
• ParallelScheduler.hpp/cpp

Detailed major modifications

Change some shared_ptr to raw pointers

added Fix 🔥🔥 label

assigned to @pineapple

mentioned in commit 81293b42

merged

mentioned in merge request !229 (merged)

mentioned in issue #199