Change return type of method connectToAP to bool + Change signature of method startSSIDscan

Similar to the methods connectSecure and connectOpen, a boolean value indicating success or failure is returned by connectToAP.

Furthermore some additional checks have been added to methods getIPAddress, ping, and getHostByName; they require the device to be connected and the DHCP process to be completed.

Method begin can return immediately if the device is already initialised.

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The return value of method startSSIDscan has been used for two purposes; the number of networks (when executed successfully) and status reporting (return false). To better distinguish between (real) results and status information, the method's return type has been changed to bool. The original return value (number of networks) is now a reference parameter.

The type of the reference parameter has been changed to uint32_t according to field num_networks of ResultStruct_t.