Fix reference return issue in MatSparse::operator().

The function prototype is imposed by the MatGeneric parent which was in its turn imposed by MatDense first function to move up in hierarchy, so we have to return a ref without having access to the buffer directly (fortunately Eigen provides the function coeffRef for that).

src/faust_linear_operator/CPU/faust_MatSparse.h

@@ -296,7 +296,7 @@ namespace Faust
 			matvar_t* toMatIOVarDense(bool transpose, bool conjugate) const;
 			FPP normL1(const bool transpose=false) const;
 			FPP normL1(faust_unsigned_int&, const bool transpose=false) const;
-			const FPP& operator()(faust_unsigned_int i, faust_unsigned_int j)const{return mat.coeff(i,j);}
+			const FPP& operator()(faust_unsigned_int i, faust_unsigned_int j)const{return const_cast<Eigen::SparseMatrix<FPP,Eigen::RowMajor>*>(&mat)->coeffRef(i,j);}
 
 			Faust::Vect<FPP,Cpu> get_col(faust_unsigned_int id) const;
 			Faust::MatSparse<FPP,Cpu>* get_cols(faust_unsigned_int col_id_start, faust_unsigned_int num_cols) const;