Various fixups (timing)

on my laptop

New implementation (nthreads=1)
Operator call with field: 0.6180651187896729
Operator call with linearization: 2.8051552772521973
Apply linearization: 0.26580238342285156
Apply linearization (adjoint): 0.26633405685424805

New implementation (nthreads=4)
Operator call with field: 0.21952056884765625
Operator call with linearization: 1.2002291679382324
Apply linearization: 0.2563960552215576
Apply linearization (adjoint): 0.25508570671081543

New implementation (nthreads=8)
Operator call with field: 0.19556593894958496
Operator call with linearization: 1.1898040771484375
Apply linearization: 0.280015230178833
Apply linearization (adjoint): 0.2798879146575928

Old implementation
Operator call with field: 1.095963478088379
Operator call with linearization: 4.0221240520477295
Apply linearization: 2.027836799621582
Apply linearization (adjoint): 4.2413599491119385

Old implementation (jax)
Operator call with field: 0.9737129211425781
Operator call with linearization: 1.2814805507659912
Apply linearization: 1.2023200988769531
Apply linearization (adjoint): 5.3104002475738525