Extract variables from analysis results across files and save as wide tables

Usage

extract_vars(params, file_rows, vars, network = FALSE, individual = FALSE, ...)

params: List of analysis parameters produced by make_params(); must contain paths$base_dir, time$intervals, and channels.
file_rows: Data frame of file metadata; must contain columns folder_path and file_id.
vars: Character vector of column names to extract from each result table.
network: Logical; if TRUE, reads network result RDS files instead of period-table RDS files. Defaults to FALSE.
individual: Logical; if TRUE, saves one row per cell instead of pivoting to a wide table. Defaults to FALSE.
...: Optional: sub (character) selects a sub-element from network results.

Called for its side effect of writing CSV tables to a tables subdirectory; returns NULL invisibly.