Multiple subroutine support

[jsrehak]

A base class needs to be added called iteration::subroutine::HasSubroutine that supports adding one or more subroutines to an inheriting class.

• The class should have a template parameter that is an enum identifying the locations that subroutines can be located.
• Subroutines are installed using AddSubroutine(LocationEnum, std::unique_ptr<SubroutineI>) that installs in particular locations.
• Provides a calling function Execute(LocationEnum, system::System) that executes the subroutine stored in the given location if the subroutine pointer is non-null.