FSL phys list plugin interface

The FSL tool gives a possibility to choose the physics list which is better for simulation by specifying the physics list name, but in some cases, there is a need to make a custom physics list.

The interface of connecting the custom physics list has been done in a similar way as a connecting of the user action plugin. For this, the new class FSLPhysListFactory was developed which can work with the physics list name, or with the physics list plugin.

After changes, the plugin can be connected through FSL as shown below in the picture, and backward compatibility is saved.

The plugin code used for the testing is attached to the description. TestPhysicsListPlugin.cc