Adds a new custom command to Softimage.
Each command must have a unique name, so this call will fail if the
name has already been registered. To avoid this sort of conflict,
you should call XSIApplication.RemoveCommand
to remove any previous registered commands, and you should try to
pick names that are unlikely to be used by Softimage itself or
other plug-in developers.
Once registered, the command becomes available to scripting, and it
can be attached to buttons on custom toolbars or added to Softimage
Menus.
Note: The recommended way to create custom commands is to define
them as part of a self-installed plug-in. See PluginRegistrar.RegisterCommand.