[Python] (scalar) vector reading speedup via numpy (#4390)

* Add numpy accessor to python flatbuffers scalar vectors

* Update python tests to test numpy vector accessor

* Update appveyor CI to run Python tests, save generated code as artifact

* Update example generated python code

* Add numpy info to python usage docs

* Update test schema and python tests w/ multi-byte vector

* did not mean to push profiling code

* adding float64 numpy tests

python/flatbuffers/compat.py

@@ -12,9 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-""" A tiny version of `six` to help with backwards compability. """
+""" A tiny version of `six` to help with backwards compability. Also includes
+ compatibility helpers for numpy. """
 
 import sys
+import imp
 
 PY2 = sys.version_info[0] == 2
 PY26 = sys.version_info[0:2] == (2, 6)
@@ -43,4 +45,37 @@ else:
         memoryview_type = memoryview
         struct_bool_decl = "?"
 
+# Helper functions to facilitate making numpy optional instead of required
+
+def import_numpy():
+    """
+    Returns the numpy module if it exists on the system,
+    otherwise returns None.
+    """
+    try:
+        imp.find_module('numpy')
+        numpy_exists = True
+    except ImportError:
+        numpy_exists = False
+
+    if numpy_exists:
+        # We do this outside of try/except block in case numpy exists
+        # but is not installed correctly. We do not want to catch an
+        # incorrect installation which would manifest as an
+        # ImportError.
+        import numpy as np
+    else:
+        np = None
+
+    return np
+
+
+class NumpyRequiredForThisFeature(RuntimeError):
+    """
+    Error raised when user tries to use a feature that
+    requires numpy without having numpy installed.
+    """
+    pass
+
+
 # NOTE: Future Jython support may require code here (look at `six`).