Add functionality to append rounds to existing config files

[annakrystalli]

This PR implements the fixes described in #42 , specifically it:

• • Adds the as_config() function to convert a list representation of a tasks.json config file to a <config> class object. Useful when wanting to programmatically manipulate the contents of a tasks.json config file.
• Adds the append_round() function to append one or more <round> class objects to the rounds property of a <config> class object.

I've also set the package version to the development version

[github-actions[bot]]

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