evangowan/paleo_sea_level: Version 2.1

I have started a new folder that stores proxy based sea level data from deep sea foraminifera records. To facilitate comparison, I have also included a few other proxies. This is separate from the main database.

I have also done a number of bug fixes and some minor changes to the Greenland data to conform to what is in the paper describing GAPSLIP 2.0 (Gowan 2023).