Merge explicit Array constructor

This makes the Array single-argument constructor explicit to avoid accidental conversions from Executor.

Related PR: #761

include/ginkgo/core/base/array.hpp

@@ -122,7 +122,7 @@ class Array {
      *
      * @param exec  the Executor where the array data is allocated
      */
-    Array(std::shared_ptr<const Executor> exec) noexcept
+    explicit Array(std::shared_ptr<const Executor> exec) noexcept
         : num_elems_(0),
           data_(nullptr, default_deleter{exec}),
           exec_(std::move(exec))