more doc work

nifty4/domains/power_space.py

nifty4/library/critical_power_energy.py

nifty4/library/nonlinear_power_energy.py

nifty4/minimization/conjugate_gradient.py

nifty4/minimization/descent_minimizer.py

nifty4/minimization/line_search_strong_wolfe.py

nifty4/minimization/minimizer.py

nifty4/minimization/nonlinear_cg.py

nifty4/minimization/vl_bfgs.py

nifty4/operators/laplace_operator.py

nifty4/operators/linear_operator.py

nifty4/operators/smoothness_operator.py