Fix mxll output

• Fix Poynting vector calculation and other outputs if broken
• Add rxExB output
• Delete maxwell_current output (in the long run we may need to add it back, discriminating by source of current)
• Delete maxwell_vector_potential output
• Delete medium_variables_electronic and medium_variables_magnetic output options
• Disable xyz output for Maxwell
• Add tests of all types of output
• Fix transversal and longitudinal fields (add warnings)
• Fix printing of fields at t=0 to output_iter (currently is zero when field is defined using user defined initial maxwell states, and this is wrong)