Changes

CoreUtilities.parseIndex added and applied to ListTag.get (#77)

* Adding CoreUtilities.parseIndex and updating ListTag.get to use it.

* Updated error handling

* Added explanations to descriptions

* Fixing list used in parseIndex call

* Formatting fixes for CoreUtilities

* Fixing based on feedback.

* Updating -1 to refer to last element in list, not second to last

* Description now accurately describes negative indices

* Changing supplied index of 0 to be an error state
 (commit: 3357984)