"FILL" algorithm:

given starting pixel in 2 or 3-d array, 
return array in which all pixels with value above threshold
and connected to that pixel by a string of pixels above threshold,
are set and all others unset.

1. go through map and set all points above threshold to 1, below to zero.
   value zero -> will never be set, value of 1 may be set, value of 2 has been set.

2. (iterative)
first cycle:  set value of the given pixel to 2, put this pixel in a list of newly set pixels.

subsequent cycles:
for each newly set pixel, check 7 or 26 neighbors (7 for 2-d, 26 for 3d).
if value of neighbor is 1, change to 2 and add to new list of newly set pixels.
After all have been checked, see if new list has any members. if so, replace old list w new list and
do another cycle.

nnew=0
do ii=1 nmembers
do ---- j=1,3
pix(j)=list(ii,j)
do ---- i=pix(1)-1, pix(1)+1       
do ---- j=pix(2)-1, pix(2)+1
do ---- k=pix(3)-1, pix(3)+1
if map(i,k,j).eq.1) then            (map(pix(1)+i,pix(2)+j,pix(3)+k).eq.1)
	nnew=nnew+1
	nlist(nnew,1)=i
	nlist(nnew,2)=j
	nlist(nnew,3)=k
continue

if (nnew.eq.0) goto end

nmembers=nnew
do ---- i=1,nmembers
do ---- j=1,3
list(i,j)=nlist(i,j)

goto start