Pvl

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* Create methods to add items to the label
* Give user option to allow the parser to succeed in parsing broken labels

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* Drastically increase test coverage.
* Lots of bug fixes.
* Add Cube and PDS encoders.
* Cleanup README.
* Use pvl specification terminology.
* Added element access by index and slice.

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* Fixed issue with reading Pancam PDS Products.

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* First release on PyPI.