when writing Measurements to a CSV, format them in a machine-readable manner

i haven't tested the ones which contains commas -- hopefully the CSV
encoder deals with these :-)

crates/post/src/bin/csv.rs

@@ -21,7 +21,7 @@ fn main() {
 
     loop {
         for meas in frame.measurements() {
-            print!("\"{}\",", meas.str_value());
+            print!("\"{}\",", meas.machine_readable().replace(",", "\\,"));
         }
         println!("");