* calc/calc.el (calc-default-power-reference-level)

* calc/calc.el (calc-default-power-reference-level)
(calc-default-field-reference-level): New variables.
* calc/calc-units.el (math-standard-units):  Add dB and Np.
(math-logunits): New variable.
(math-extract-logunits, math-logcombine, calcFunc-luplus)
(calcFunc-luminus, calc-luplus, calc-luminus, math-logunit-level)
(calcFunc-fieldlevel, calcFunc-powerlevel, calc-level): New
functions.
(math-find-base-units-rec): Add entry for ln(10).
* calc/calc-help.el (calc-u-prefix-help): Add logarithmic help.
(calc-ul-prefix-help): New function.
* calc/calc-ext.el (calc-init-extensions): Autoload new units
functions. Add keybindings for new units functions.