Describe use of precision tolerances

Signed-off-by: Håkon Wiik Ånes <hwaanes@gmail.com>

orix/constants.py

@@ -1,3 +1,20 @@
+# Copyright 2018-2024 the orix developers
+#
+# This file is part of orix.
+#
+# orix is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# orix is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with orix.  If not, see <http://www.gnu.org/licenses/>.
+
 """Constants and such useful across modules."""
 
 from importlib.metadata import version
@@ -12,7 +29,9 @@
     except ImportError:
         installed[pkg] = False
 
-# Tolerances
+# Typical tolerances for comparisons in need of a precision. We
+# generally use the highest precision possible (allowed by testing on
+# different OS and Python versions).
 eps9 = 1e-9
 eps12 = 1e-12