Topological order

The idea is to sort addins in a topological order. Each addin can define one of five things for _order_:
* `>X` - I must be immediately after the addin with id X

* `>>X` - I must be somewhere after the addin with id X (but not necessarily immediately) 

* `<X`- I must be immediately before the addin with id X

* `<<X` - I must be somewhere before the addin with id X (but not necessarily immediately) 

* `Number` - A numerical order which is used if none of the above is applicable


i suggest pre-validating your cluster inputs. you won't need this test, and others in this function, and it will be more readable, maintainable and testable.
good functions are cohesive - they do one thing. this function does several things, it validates, parses, and has applicative logic.