Expander last_message_age property.

[SeaTechRC]

Hello,

This change enables one to write rules or higher level controls which are dependent on wether or not the expander actually transmits data.
This could be used for motor-movement based on sensor data from a sensor on an expander, where communication could fail or is highly time-critical.

[falkoschindler]

Thanks for the pull request, @SeaTechRC! The "last_message_age" property is a great idea!

I just added it to the documentation and removed some trivial comments.
And I think this->last_message_millis = millis(); can be moved out of the if-condition checking for "!!". This condition is only for distinguishing broadcast messages that need to be evaluated as Lizard code and other messages that will simply be printed to the terminal. last_message_millis should always be updated, independent of the message type or content.

@Johannes-Thiel Can you briefly check this new feature with a Robot Brain? Thanks!

[Johannes-Thiel]

I did a test on the testbrain and the result was as expected.