Improved ClusterFile API (#358)

Improved API for ClusterFile

- read_frame now returns a std::optional<ClusterVector> to differentiate
between an empty frame and end of file
- f.frames() produces an iterator over frames
- f.chunks() produces an iterator reading chunks of clusters

src/FilePtr.test.cpp

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#include "aare/FilePtr.hpp"
+
+#include <catch2/catch_test_macros.hpp>
+#include <utility>
+
+TEST_CASE("FilePtr converts to its open state", "[FilePtr]") {
+    aare::FilePtr empty;
+    CHECK_FALSE(empty);
+
+    aare::FilePtr open(__FILE__, "rb");
+    CHECK(open);
+
+    aare::FilePtr moved(std::move(open));
+    CHECK(moved);
+    CHECK_FALSE(open);
+}