Marks the associated element as being only available on a certain 
Side. This is
 generally meant for internal Forge and FML use only and should only be used on mod classes
 when other more common mechanisms, such as using a 
SidedProxy fail to work.
 Note, this will 
only apply to the direct element marked. This code:
 
 @SideOnly public MyField field = new MyField(); will 
not work, as the initializer
 is a separate piece of code to the actual field declaration, and will not be able to find
 it's field on the wrong side.