Updating cpu backend for bitshift with the new rounding attribute

Adapting backend_cpu [BitShiftImpl_kernels.hpp] and units test to use the new rounding attribute of bitshift operator as described in this issue [aidge_core#239 (closed)\]

include/aidge/backend/cpu/operator/BitShiftImpl.hpp

@@ -24,6 +24,7 @@ namespace Aidge {
 // Operator implementation entry point for the backend
 using BitShiftImpl_cpu = OperatorImpl_cpu<BitShift_Op,
     void(const BitShift_Op::BitShiftDirection,
+    const bool,
     std::vector<std::size_t>,
     std::vector<std::size_t>,
     const std::vector<std::size_t>&,