fix plotting for fields with both electrostatic and magnetostatic components

[jobdewitte]

Problem statement

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Right now, plotting with plot_equipotential_lines can only be done for fields with either a magnetic or electric component, due to the usage of potential_at_point in line 247 of plotting.py. Otherwise an error is raised

Proposed Solution

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Give the user the an option to set which component of the field should be plotted via a keyword in the plot_equipotential_lines function.