Removed the (apparently unused) 'factor' and 'offset' arguments from

Removed the (apparently unused) 'factor' and 'offset' arguments from
property-adjust.

Modified property-adjust and property-multiply so that they always
work with double values, to simplify the code.

Factored out some common code.

Added a 'mask' argument to the property-adjust and property-multiply
commands.  If mask is set to "all" (the default), then the command
works as before; if mask is set to "integer", it works only to the
left of the decimal point; and if mask is set to "decimal", it works
only to the right of the decimal place.  This functionality is useful
for tuning VHF COM and NAV radios.