* calc/calc-units.el (math-midi-round, math-freqp, math-midip)

* calc/calc-units.el (math-midi-round, math-freqp, math-midip)
  (math-spnp, math-spn-to-midi, math-midi-to-spn, math-freq-to-spn)
  (math-midi-to-freq, math-spn-to-freq, calcFunc-spn, calcFunc-midi)
  (calcFunc-freq, calc-freq, calc-midi, calc-spn): New functions.
  (math-notes): New variable.

* calc/calc.el (calc-note-threshold): New variable.

* calc/calc-ext.el (calc-init-extensions): Add keybindings for
  calc-spn, calc-midi, calc-freq.  Add autoloads for calcFunc-spn,
  calcFunc-midi, calcFunc-freq, calc-spn, calc-midi and calc-freq.

* doc/misc/calc.tex (Musical Notes): New section.
  (Customizing Calc): Mention calc-note-threshold.