Added NodeList index to DataBase

[brbass]

This lets us calculate the index in the DataBase that represents a given NodeList in the connectivity.

ToDo :

• Annotate RELEASE_NOTES.md with notable changes.
• Create LLNLSpheral PR pointing at this branch. (PR#)
• LLNLSpheral PR has passed all tests.

[jmikeowen]

This looks fine. A heads up for the future -- I've been slowly transitioning things like this from int to size_t, which I now know is the more appropriate type for sizes and indices into containers like this. The int here is fine for now though, and consistent with the other things that return int's in the DataBase interface.

[mdavis36]

@brbass I know you didn't want to create a separate LLNL PR for this. However, if you could have #PR113 point to this branch we can consider the LLNLSpheral requirement for this PR satisfied when both pass.

[brbass]

@brbass I know you didn't want to create a separate LLNL PR for this. However, if you could have #PR113 point to this branch we can consider the LLNLSpheral requirement for this PR satisfied when both pass.

That MR is pointing at this one already. We should wait until that one is ready in case I need to make other changes.