ete_diff.py: Do not immediately fail if lap module not present.

Otherwise we cannot even run the
  ete4 ...
script in installations where everything else is fine.

ete4/tools/ete_diff.py

@@ -10,7 +10,13 @@
 import multiprocessing as mp
 from ..coretype.tree import Tree
 from ..utils import print_table, color
-from lap import lapjv
+
+try:
+    from lap import lapjv
+except ImportError:
+    def lapjv(*args, **kwargs):
+        raise ValueError('lapjv could not be imported.')
+
 import textwrap
 import argparse
 import logging