Added '''ActionList.length''' function.

Added `length()` function to `ActionList` which returns number of commands added to an instance of `ActionList`. It should be accessed as a function not as a property: `myActionList.length();`
`Frontier` will throw an error if unhanded topic is invoked
Added tests
Updated versions
Merge branch 'feature/unhandled-topics' into develop
Merge branch 'develop'