Improve integration of expander module

[falkoschindler]

• [x] When instantiating an expander module, the execution should wait until the expander has sent a Ready message.
• [x] When instantiating a proxy module, the execution should wait until the expander has sent a first broadcast message (see #66).
• [x] 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.
• [x] A new last_message_age property can be used to define safety rules that stop the machine on connection loss.
• [x] Expressions for reading properties need to be evaluated at runtime, not at compile time (see PR #51).
• [x] Some kind of heart beat message should make sure a lost connection to the expander is detected.