Fix bugs in snack_exe()
Fix bugs in snack_exe()

* Make sure snack command options (e.g. windowlength) are only used if
  they are specified.  Otherwise, they should be left off.
* Raise error if .f0 file expected is not produced by snack.exe

voicesauce/opensauce-python,voicesauce/opensauce-python,voicesauce/opensauce-python