GetFileSystemUsage(...) returns Kilobytes instead of Bytes

GetFileSystemUsage returns Kilobytes instead of Bytes.

Example Code which should print the used Gigabytes:

package main

import (
	"fmt"

	sigar "github.com/cloudfoundry/gosigar"
)

func main() {
	s := sigar.ConcreteSigar{}
	fsUsage, _ := s.GetFileSystemUsage("/System/Volumes/Data")

	gbUsed := float64(fsUsage.Used) / 1024 / 1024 / 1024  // converting bytes to Gigabytes
	fmt.Printf("%3.2f GB used\n", gbUsed)
}

Result on my machine:

0.34 GB used

Expected result:

> df -h
Filesystem       Size   Used  Avail Capacity iused      ifree %iused  Mounted on
...
/dev/disk3s5    460Gi  338Gi  108Gi    76% 6446774 1131120840    1%   /System/Volumes/Data
...

OS: macOS
gosigar version: 1.3.36

Start by locating the macOS-specific implementation of GetFileSystemUsage and trace how its Used value is obtained and represented. Compare the returned unit with the macOS df output shown in the issue. Done means the API reports Used in bytes, so the example's conversion produces a value consistent with df.