Replace pandas with numpy.testing

Addresses https://github.com/jensengroup/propka-3.1/pull/36#discussion_r429104797

requirements.txt

@@ -1,2 +1,2 @@
 pytest
-pandas
+numpy

setup.py

@@ -57,6 +57,6 @@ See http://propka.org/ for the PROPKA web server.
         },
       zip_safe=True,
       python_requires='>=3.5',
-      tests_require=["pandas", "pytest"],
+      tests_require=["pandas", "numpy"],
       test_suite="tests",
 )

tests/README.md

@@ -1,6 +1,7 @@
 # Testing PROPKA
 
-These tests assume that PROPKA is installed as a module on your system.
+These tests assume that PROPKA is installed as a module on your system the 
+dependencies in `../requirements.txt` are satisfied.
 If you are running in a virtual environment and want to make changes to your
 code, module installation accomplished by
 ```

tests/test_basic_regression.py

@@ -4,7 +4,7 @@ import os
 import re
 from pathlib import Path
 import pytest
-import pandas as pd
+from numpy.testing import assert_almost_equal
 import propka.lib
 import propka.molecular_container
 
@@ -12,9 +12,10 @@ import propka.molecular_container
 _LOGGER = logging.getLogger(__name__)
 
 
-# Maximum error set by number of decimal places in pKa output as well as need
-# to make unmodified code work on WSL Ubuntu 18.04
-MAX_ERR = 0.01
+# Number of decimal places for maximum tolerable error.  Set by number of
+# decimal places in pKa output as well as need to make unmodified code work
+# on WSL Ubuntu 18.04
+MAX_ERR_DECIMALS = 2
 
 
 # This directory
@@ -102,13 +103,9 @@ def compare_output(pdb, tmp_path, ref_path):
                 m = re.search(r'([0-9]+\.[0-9]+)', line)
                 value = float(m.group(0))
                 test_data.append(value)
-
-    df = pd.DataFrame({"reference": ref_data, "test": test_data})
-    df["difference"] = df["reference"] - df["test"]
-    max_err = df["difference"].abs().max()
-    if max_err > MAX_ERR:
-        errstr = "Error in test (%g) exceeds maximum (%g)" % (max_err, MAX_ERR)
-        raise ValueError(errstr)
+    errstr = "Error exceeds maximum allowed value (%d decimal places)" % MAX_ERR_DECIMALS
+    assert_almost_equal(test_data, ref_data, decimal=MAX_ERR_DECIMALS,
+                        err_msg=errstr, verbose=True)
 
 
 @pytest.mark.parametrize("pdb, options", [