Hotfix floating point representation error discovered in angle2()

Still seeing some errors so reduced the COLIN_TOLERANCE from 13 to 10.
This solves the specific example given in #33 and I also mentioned
in #28 that I would reduce the tolerance to better avoid these types
of errors.

@tjansson60 has a suggested solution that does not involve truncating
the cos_value, I need to performance test this first.

See: #33, #28, #19

pyvisgraph/visible_vertices.py

@@ -28,7 +28,7 @@
 INF = 10000
 """Due to floating point representation error, some functions need to
    truncate floating point numbers to a certain tolerance."""
-COLIN_TOLERANCE = 13
+COLIN_TOLERANCE = 10
 T = 10**COLIN_TOLERANCE
 T2 = 10.0**COLIN_TOLERANCE