Fix infinite loop in TimeMarks::add_time_marks() caused by zero dt.
- In that case only the initial time mark is added.
Implemented methods in Input::Type::Selection.
- copy_values() to add values from another selection
- int_to_name() to access value names by id
Changed input format for equation output.
- primary and secondary equation specify their output stream and an array output_fields indicating which fields will be saved
- equation informs the output stream about fields that are to be saved
- key save_step renamed to time_step and moved to output_stream record