Setup phases

[pauxus]

in addition to #155 and #146 we could structure the whole setup into phases:

• create
• apply
• validate
• use

this would allow central structuring of the lifecycle of an model, help with readable error and validation messages and make the code cleaner.

However, a phase mechanism might make it necessary to include more classes in the classpath

[pauxus]

Phase apply defaults (before or after validate?)

[pauxus]

Apply is no separate phase, since apply is done per object and not per tree