custom icon directory should have a relative path #129
Description
When moving an experiment with custom icons to a different computer, the path has to be set again. It would be easier to have icon folder paths relative, so that doesn't have to be done. Of course, this only works if the icon folder that is used is part of the PyVMEC folder, like the custom_icon folder it comes with. But it would still make life easier. It would be even better if those relative paths are also platform independent.