Synchronize start/end of method with subtasks

[arbimo]

Enforce that the start/end of a method equal to the start/end of earliest/latest subtask.

Currently we only enforce that all subtasks are contained within the methods. However a method could "start" before its first subtask This is unsound on partial-order domains with method preconditions.

• use max/min constraints to enforce equality (requires Add CSP module with arbitrary propagators #19)
• opimization: analyze the temporal constraints in the method to use the start/end timepoint of the method as the start/end of the first/ast subtask (thus reducing the number of timepoints in the encoding)