added array::ensure() function wrapping PyArray_FromAny

This convenience function ensures that a py::object is either a
py::array, or the implementation will try to convert it into one. Layout
requirements (such as c_style or f_style) can be also be provided.

include/pybind11/numpy.h

@@ -425,6 +425,13 @@ public:
         return array(api.PyArray_Squeeze_(m_ptr), false);
     }
 
+    /// Ensure that the argument is a NumPy array
+    static array ensure(object input, int ExtraFlags = 0) {
+        auto& api = detail::npy_api::get();
+        return array(api.PyArray_FromAny_(
+            input.release().ptr(), nullptr, 0, 0, detail::npy_api::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr), false);
+    }
+
 protected:
     template<typename, typename> friend struct detail::npy_format_descriptor;
 
@@ -466,7 +473,7 @@ protected:
 
 template <typename T, int ExtraFlags = array::forcecast> class array_t : public array {
 public:
-    PYBIND11_OBJECT_CVT(array_t, array, is_non_null, m_ptr = ensure(m_ptr));
+    PYBIND11_OBJECT_CVT(array_t, array, is_non_null, m_ptr = ensure_(m_ptr));
 
     array_t() : array() { }
 
@@ -518,7 +525,7 @@ public:
 
     static bool is_non_null(PyObject *ptr) { return ptr != nullptr; }
 
-    static PyObject *ensure(PyObject *ptr) {
+    static PyObject *ensure_(PyObject *ptr) {
         if (ptr == nullptr)
             return nullptr;
         auto& api = detail::npy_api::get();