Should rename accumulate_Hessian_times_input and add_multiplication_with_approximate_Hessian

[robbietuk]

These two methods both compute the multiplication of a (approximate) Hessian and a vector. This is commonly refered to as a Hessian_vector_product. Renaming the current methods was first discussed here https://github.com/UCL/STIR/pull/901#discussion_r672560011

I suggest the following renames:

• accumulate_Hessian_times_input -> accumulate_Hessian_vector_product, and
• add_multiplication_with_approximate_Hessian -> accumulate_approximate_Hessian_vector_product

Chanegs would need to be made for both data fit terms and priors, including their implementations in OSSPS and SqrtHessianRowSum

[robbietuk]

Related to some comments in #718