Add ensure_continguous_memoryview function

Include a function to ensure an object is converted into a contiguous
`memoryview` object.

Author: jakirkham

numcodecs/compat_ext.pxd

@@ -0,0 +1,4 @@
+# cython: language_level=3
+
+
+cpdef memoryview ensure_continguous_memoryview(obj)

numcodecs/compat_ext.pyx

@@ -0,0 +1,19 @@
+# cython: embedsignature=True
+# cython: profile=False
+# cython: linetrace=False
+# cython: binding=False
+# cython: language_level=3
+
+from cpython.buffer cimport PyBuffer_IsContiguous
+from cpython.memoryview cimport PyMemoryView_GET_BUFFER
+
+
+cpdef memoryview ensure_continguous_memoryview(obj):
+    cdef memoryview mv
+    if type(obj) is memoryview:
+        mv = <memoryview>obj
+    else:
+        mv = memoryview(obj)
+    if not PyBuffer_IsContiguous(PyMemoryView_GET_BUFFER(mv), b'A'):
+        raise BufferError("Expected contiguous memory")
+    return mv

setup.py

@@ -276,6 +276,23 @@ def jenkins_extension():
     ]
 
 
+def compat_extension():
+    info('setting up compat extension')
+
+    extra_compile_args = base_compile_args.copy()
+
+    sources = ['numcodecs/compat_ext.pyx']
+
+    # define extension module
+    return [
+        Extension(
+            'numcodecs.compat_ext',
+            sources=sources,
+            extra_compile_args=extra_compile_args,
+        ),
+    ]
+
+
 def shuffle_extension():
     info('setting up shuffle extension')
 
@@ -344,6 +361,7 @@ def run_setup(with_extensions):
             blosc_extension()
             + zstd_extension()
             + lz4_extension()
+            + compat_extension()
             + shuffle_extension()
             + vlen_extension()
             + fletcher_extension()