Allow ExaTron to use Hessian-vector product

[frapac]

An idea is to implement Hessian-vector product as a linear operator, such as

struct HessVecProd <: AbstractTronMatrix 

(maybe we could rename AbstractTronMatrix as AbstractTronLinearOperator). Then by overloading the * operator,
we could apply Hessian-vector product as with the other types (TronDenseMatrix and TronSparseMatrixCSC), simply as H * v. In that case, we would keep the clean splitting we have between the evaluators (called only in driver.jl) and the Tron algorithm.

Would require to merge #12 first, as we won't be able to use ICFS in this case