Only update is_charge_density when explicitely given
Commit c51ebff5 added an is_charge_density option to LinearProblem.update(), which causes an unintended change of behavior in old code: when is_charge_density is not passed as an argument it is still set to True.
This MR makes sure that is_charge_density is only updated when explicitly passed as an argument, such that the module is compatible with both old (unchanged) code and newly written code.