global variable reorganization

[suz-estella]

I've added a section to our package defining R file R/CBMutils-package.R that declares commonly used variables with utils::globalVariables between all of our functions. These were often declared in multiple files, or used in multiple files but only declared in some, etc. This will make it easy to use these variables anywhere.

This includes:

1. data.table special variables (e.g. :=)
2. Common spatial variables (e.g. x)
3. Common keys (e.g. pixelIndex)
4. Boudewyn table columns (e.g. juris_id)
5. CBM defaults database columns (e.g. spatial_unit_id)
6. CBM-EXN cbm_vars columns (e.g. row_idx)

We can add to this over time as we see fit.