Add secondary objective function

The secondary objective function is used to choose amongst all the solutions that minimise the primary objective function.

Only the user should add terms to the primary objective function. The component/sequence developer can add terms to the secondary objective function.

There should be 4 levels of cost functions, which are optimised in order:

1. any cost function that should take precedence over all user-set goals.
2. the default level for any goals set by the user.
3. the default level for any goals set by the developer.
4. any secondary goals, that should only be fulfilled if they do not conflict with any other goals.

created branch 10-add-secondary-objective-function to address this issue

mentioned in merge request !4 (merged)

closed with commit 454b9a60