ComplexVector.h: RowVector class variables childrenSize_ and children_ keywords discrepancy

[minhancao]

Description

In ComplexVector.h RowVector class, the childrenSize_ variable is defined as const but the children_ variable is defined as mutable...

const size_t childrenSize_;
mutable std::vector<VectorPtr> children_;

We should make both to be the same to be either const or mutable.