Improve integration of expander module

[falkoschindler]

• When instantiating an expander module, the execution should wait until the expander has sent a Ready message.
• When instantiating a proxy module, the execution should wait until the expander has sent a first broadcast message.
• Later we could add constructor parameters for skipping the wait to save time when working with many expanders.
• New methods like expander.await_ready() or proxy.await_broadcast() allow awaiting many expanders at once.
• A new last_broadcast_age property can be used to define safety rules that stop the machine on connection loss.
• Expressions for reading properties need to be evaluated at runtime, not at compile time (see PR Introduce property expression to avoid evaluating properties too early #51).
• Some kind of heart beat message should make sure a lost connection to the expander is detected.