function: write_reg -- save to register.dat
keyboard equivalent: none

The SAVE REG button saves the currently visible
registration (and the current sample method) to
the register.dat file.

To change the sample method (round vs. trunc),
access the larger interface with F3 (linux) or
fn-F3 (Mac) and read the help for the "round"
checkbutton (lower right).  The F3 panels also
contains READ REG, which can be used to revert
the registration to the last saved register.dat.
