Returns an input stream associated with the resource referenced
by filename. If DLSim is being executed as an applet, the
resource directory is the URL whose path is the same as the
circuit containing the plugin. If DLSim is being executed
as an application, the resource directory is the current
value of the system property "user.dir".
In order to be able to execute circuits in either
application or applet mode, plugin authors are advised to
use getInputStream to retrieve file data used in their
circuits.