factor out view code into ElectricPotentialLineView, make potential line view and model more mutable, https://github.com/phetsims/charges-and-fields/issues/170
factor out view code into ElectricPotentialLineView, make potential line view and model more mutable, https://github.com/phetsims/charges-and-fields/issues/170

phetsims/axon,phetsims/axon,phetsims/axon