Add proper sample rate detection to transcode.py

88.2 kHz ᴏʀ 176.4 kHz  ᴛᴏ 44.1 kHz
96 kHz ᴏʀ 192 kHz ᴄᴏɴᴠᴇʀᴛs ᴛᴏ 48 kHz

See https://what.cd/wiki.php?action=article&id=683