Objective functions should not compute the sensitivities upon `set_up`

[robbietuk]

Utilities such as OSSPS,compute_sqrt_Hessian_row_sum, demo4 and demo5 utilise Poisson objective functions but do not need the system matrix sensitivities (A^T 1), yet they are computed. This additional and unnessicery computation slows down these utilities.

Perhaps with the changes being made in #893, the sensitivity do not need to be computed during set_up. Prior to this PR, the sensitivities were used in the subset-gradient computation, grad = A_m^T [ y_m/ (A_m(x) + b_m) ] - sensitivity_m making this suggestion infeasable.

When #893 is merged, it could be that the sensitivities are only computed if objective_function.get_subset_sensitivity(subset_num); is called.