Add descriptive labels to data fields

[jburos]

There are a handful of labelled fields, but some additional labels would be helpful.

For example, in the subjects table:

1. subject_id: universal subject identifier (UUID), unique within the database (varies across projectversions)
2. individual_id: subject id provided in the dataset; may not be unique within a project
3. pdl1tc: category of pdl1 expression on tumor cells
4. pdl1ic: category of pdl1 expression on immune cells

[jburos]

labels for subjects data are here: https://github.com/generable/rgeco/blob/main/R/geco_subjects.R#L113-L123