need to add .pop() and .remove() to LiPD object

[khider]

Need to have some ways to remove records from a LiPD object, given a dataset name

• .pop() should pop is out into a new object
• .remove() should just remove from the object either in place, or by creating a new object.

[varunratnakar]

Added with https://github.com/LinkedEarth/pylipd/commit/6817dd080e0c61bca677e8c6ec351b3eca9759f6