Increase cell_drift max -- improves decoding with distortion

The explicit `undistort` step we can run as part of the extract is not
particularly great, so making the decoder smarter is useful. Also, it
turns out that the flood decode does a pretty good job staying on the
rails.

Author: sz3

.gitmodules

@@ -2,3 +2,4 @@
 	path = samples
 	url = https://github.com/sz3/cimbar-samples
 	branch = v0.5
+	ignore = dirty

cimbar/encode/cell_positions.py

@@ -4,7 +4,7 @@
 
 class cell_drift:
     pairs = [(0, 0), (1, 0), (0, 1), (-1, 0), (0, -1), (1, 1), (-1, -1), (1, -1), (-1, 1)]
-    limit = 2
+    limit = 7
 
     def __init__(self, x=0, y=0):
         self.x = x