Disallow running neuclease if numexpr is installed.

See #3

neuclease/__init__.py

@@ -24,3 +24,33 @@ def configure_default_logging():
     root_logger.handlers = []
     root_logger.addHandler(handler)
     root_logger.setLevel(logging.INFO)
+
+try:
+    import numexpr
+except ImportError:
+    pass
+else:
+    import textwrap
+    msg = textwrap.dedent("""\
+        Cannot use this library in an environment in which numexpr is installed.
+
+        Please do not use this library in an environment in which numexpr is installed.
+
+        This library relies on pandas eval() and pandas query() to compare uint64 types,
+        which is not supported by the numexpr library.
+        
+        If numexpr is present, pandas will use it by default in eval() and query(),
+        rather than using query(..., parser='python').
+        
+        Using numexpr leads to errors like this one:
+
+            >>> df = pd.DataFrame([[1,2]], columns=list('ab'), dtype=np.uint64)
+            >>> df.query('a == b')
+            TypeError: Iterator operand 1 dtype could not be cast from dtype('uint64') to dtype('int64') according to the rule 'safe'
+        
+        In the future, this library will be patched to avoid using uint64 types altogether,
+        and this error will be removed.
+        
+        See: https://github.com/janelia-flyem/neuclease/issues/3
+    """)
+    raise AssertionError(msg)