Add Interval Option

Add a linux-like interval option to set the time to wait between pings, in seconds. Thanks to @deltronzero for the PR and the idea!

This is a BREAKING change to some extent. If you are using positional parameters, the interval option was added BEFORE some existing parameters, so you would end up filling the wrong parameter with your data. Specifically:

• If you are calling ping() and providing payload, sweep_start or sweep_end positionally, you need to update your code to provide interval first (default value is zero).
• If you are instantiating the Communicator class and providing either socket_options, seed_id, verbose, or output positionally, then you need to update your code to include interval first (also here, default value is None but zero is fine as well).