variable: $roundregflag -- sample round (vs. trunc)
default: none -- read from register.dat

When tkregister reads a register.dat file
containing a line, "round", following the
transformation matrix, the "round" checkbutton is
checked ($roundregflag=1) and tkregister rounds
the transformed floating point position before
nearest-neighbor sampling the moveable volume.

If register.dat lacks "round" (or contains
"tkregister"), then the original behavior of
sampling by truncating the transformed floating
point position is used.

In both cases, the overlay visible in tkregister
will be properly sampled to the surface when
PAINT is clicked in a later panel (i.e., paint.c
also changes its behavior depending on whether or
not register.dat "round" is present).

If the state of the "round" checkbutton is
changed after opening tkregister, the moveable
display will be updated (the moveable image will
move half a voxel in all three axes).  The first
time that register.dat is saved with SAVE REG
after such a change, there will be a popup
warning that the sampling method is being
changed.

The "round" line in register.dat affects the
behavior of tksurfer (rawdata sample) and tkmedit
(rawdata sample, stats overlay) similarly.
