SymbolicBlockMatrix memory leak

[jorisv]

SymbolicBlockMatrix store two raw pointers.

These pointers are not owner by SymbolicBlockMatrix and this create leaks in unit tests and benchmarks.

Two options :

1. Make SymbolicBlockMatrix owner of these two pointers
2. Create SymbolicBlockMatrixHandler that will own a SymbolicBlockMatrix and these two pointers

Since these two pointers are only used by SymbolicBlockMatrix I prefer option 1.