Returns the Hertz value of freq, which can be a note, keynum,
Hertz value or list of the same. To specify freq in Hertz
add the symbol tag :hertz or :hz immediately
after the value.
If :in is specified it sets the scale for resolving a note or keynum freq.
The value of :in defaults to the global variable
*scale*, which is initially set to the standard chromatic scale.
The read macro #h provides a shorthand for converting notes or keynums to Hertz.