tests: report only relevant differences for dpre

[landam]

Current diff style:

--- 
+++ 
@@ -1,6 +1,6 @@
 NoDataValue:-9999
 STREAM_RATIO:None
-array_points:[[ 1.      1.      2.      1.1635  7.3094]
 [ 2.     13.      2.      1.3745  1.4116]
 [ 3.     15.      4.      2.2624  0.2335]
 [ 4.     15.      0.      0.2573  0.2646]
 [ 5.     15.      3.      1.8487  0.2891]
 [ 7.      0.      2.      1.1193  7.8153]]
+array_points:[[ 1.          1.          2.          1.16349162  7.30944235]
 [ 2.         13.          2.          1.37450624  1.4115917 ]
 [ 3.         15.          4.          2.26240022  0.23353809]
 [ 4.         15.          0.          0.257301    0.26458773]
 [ 5.         15.          3.          1.84874634  0.28907763]
 [ 7.          0.          2.          1.11926547  7.81530241]]
 bc:None
 br:None
 c:5

New diff style:

--- output (array_points)
+++ reference (array_points)
@@ -1 +1 @@
-array_points:[[ 1.      1.      2.      1.1635  7.3094]
 [ 2.     13.      2.      1.3745  1.4116]
 [ 3.     15.      4.      2.2624  0.2335]
 [ 4.     15.      0.      0.2573  0.2646]
 [ 5.     15.      3.      1.8487  0.2891]
 [ 7.      0.      2.      1.1193  7.8153]]
+array_points:[[ 1.          1.          2.          1.16349162  7.30944235]
 [ 2.         13.          2.          1.37450624  1.4115917 ]
 [ 3.         15.          4.          2.26240022  0.23353809]
 [ 4.         15.          0.          0.257301    0.26458773]
 [ 5.         15.          3.          1.84874634  0.28907763]
 [ 7.          0.          2.          1.11926547  7.81530241]]

[landam]

Questions:

• rel / abs tol ?
• compute diffs only for reported variables?