Loosen error tolerance in test.

The PROPKA31 tests fail on Windows 10 Ubuntu 18.04 Windows Subsystem
Linux without any changes.  The relative error is less than 1e-3.  This
commit chnages the behavior to a test of absolute agreement to a test of
relative error less than 1e-3.

Tests/pkacalc_test.py

@@ -4,6 +4,11 @@ import re
 from subprocess import call
 import sys
 import unittest
+import logging
+
+
+ACCEPTABLE_ERROR = 0.001
+
 
 # Setting this up as a direct translation of the original runtest.py script
 # that will be run as part of 'python setup.py test'. This takes on the
@@ -53,8 +58,10 @@ class SystemTest(unittest.TestCase):
 
                 expected_value = float(ref.readline())
                 value = float(m.group(0))
+                value_error = (value-expected_value)/expected_value
 
-                if(value != expected_value):
+                if abs(value_error) > ACCEPTABLE_ERROR:
+                    logging.error(value_error)
                     identity = line[:m.start()].strip()
                     errors.append("%12s  %8.2f    %8.2f" %
                             (identity, expected_value, value))