Ion functions

## Adds a new submodule for marine science functions

This will require a buildout to accomodate the changes. Very important component is that developers will need to include the libgswteos-10 library:
```bash
brew tap lukecampbell/hombrew-libgswteos
brew install libgswteos-10
brew test -v libgswteos-10
```
A subsequent buildout will include all of the necessary libraries. 

This submodule will host a repository for authors to define and manage functions to execute marine science calculations. These are required to calculate the marine seawater variables used ultimately for deriving data products from instrument variables.