empymod/emg3d: Complex sources

Strength input for get_source_field can now be complex; it also stores now the source location and its strength and moment. get_receiver can now take entire Field instances, and returns in that case (fx, fy, fz) at receiver locations. Krylov subspace solvers: Solver now finishes in the middle of preconditioning cycles if tolerance is reached. Solver now aborts if solution diverges or stagnates also for the SSL solvers; it fails and returns a zero field. Removed gmres and lgmres from the supported SSL solvers; they do not work nice for this problem. Supported remain bicgstab (default), cgs, and gcrotmk. Various small things: New attribute Field.is_electric, so the field knows if it is electric or magnetic. New verb-possibility: verb=-1 is a continuously updated one-liner, ideal to monitor large sets of calculations or in inversions. The returned info dictionary contains new keys: runtime_at_cycle: accumulated total runtime at each cycle; error_at_cycle: absolute error at each cycle. Simple __repr__ for TensorMesh, Model, Fourier, Time. Bugfixes: Related to get_hx_h0, data_write, printing in Fourier