Allow arbitrary functional selector for expectation

[ThomasDebrunner]

This adds the following expectaction prototype:

Expectation expect(std::function<bool(const mav::Message&)> selector)

Which can be used for specifc queries, e.g.

auto expectation = connection.expect([](const mav::Message &message) {
   return message.id() == 11232 && message["field3"].as<int>() == 15;
})

auto message = connection.receive(expectation);

The current expect functions

Expectation expect(int message_id, int source_id=mav::ANY_ID, int component_id=mav::ANY_ID)
Expectation expect(const std::string &message_name, int source_id=mav::ANY_ID, int component_id=mav::ANY_ID)

Have been changed to be specializations of the more generic expect function with the funcional argument

This also adds a new receive prototype:

Message inline receive(std::function<bool(const mav::Message&)> selector, int timeout_ms=-1)

Which again gives the user the possibility to receive a very specific message. This also makes use of the new expect function.