rarray: bound spacings to greater than 0 (#454)

* rarray: bound spacings to greater than 0
This addresses Issue 451

* Fix a typo. (#452)

* Fix a typo.

* Pin black version in travis

Co-authored-by: Adam Urbańczyk <adam-urbanczyk@users.noreply.github.com>

* rarray: bound spacings to greater than 0
This addresses Issue 451

* Added test for rarray error handling

Co-authored-by: Kevin Menard <kevin@nirvdrum.com>
Co-authored-by: Adam Urbańczyk <adam-urbanczyk@users.noreply.github.com>

tests/test_cadquery.py

@@ -8,7 +8,7 @@ from random import choice
 from random import random
 from random import randrange
 
-from pytest import approx
+from pytest import approx, raises
 
 # my modules
 
@@ -960,6 +960,9 @@ class TestCadQuery(BaseTest):
         # 6 faces for the box, 2 faces for each cylinder
         self.assertEqual(6 + NUMX * NUMY * 2, s.faces().size())
 
+        with raises(ValueError):
+            Workplane().rarray(0, 0, NUMX, NUMY, True)
+
     def testPolarArray(self):
         radius = 10