Method to retrieve Arbor configuration in C++ / cleanup of busyring example

[jlubo]

This PR introduces a method to get a string representation of the Arbor configuration in C++ (analogously to the config() method that has already been available for the Python frontend).

While the C++ function was already introduced by PR #2489 within the busyring example, this PR makes it globally available as a member of the arb class.

The PR further introduces small fixes for the busyring example (in the most recent version, the synapse number was off by 1) and for the profiler (missing header file).

[thorstenhater]

Hi @jlubo,

thank you, having the configuration as a table of values at runtime is really helpful. However, I am not sure though whether locking this to a string is necessarily the most useful way of implementing the functionality. My feeling is
that providing a map<string, value> where value = variant<string, int> or possibly just value = string and having a pretty printer on top of that might be more natural. What do you think?