empymod/emg3d: Zero Source

Bug fixes:

• Checks now if provided source-field is zero, and exists gracefully if so, returning a zero electric field. Until now it failed with a division-by-zero error.

Improvements:

• Warnings: If verb=1 it prints a warning in case it did not converge (it finished silently until now).
• Improvements to docs (figures-scaling; intersphinx).
• Adjust Fields.pha and Fields.amp in accordance with empymod v2: .pha and .amp are now methods; uses directly empymod.utils.EMArray.
• Adjust tests for empymod v2 (Fields, Fourier).