Refactor times.py

[cleder]

Refactor times to use the registry.

• Add new classes When, Begin and End
• Remove the etree_element and _get_kwargs methods from TimeStamp and TimeSpan
• Add new function following the GetKWArgs Protocol to parse the KmlDateTime from XML
• Add new function following the SetElement Protocol to create the etree representation
• Register the functions for the new classes
• TimeStamp to optionally take the above when
• TimeSpan to optionally take begin and end

[rishitc]

Hi @cleder, I'm interested in working on this issue. Can you please assign it to me?

Thanks

[cleder]

Thanks @rishitc