[BL] Very simple dynamic span class.

Should be replaced with std::span when the compiler version
gcc >= 10, msvc >= 19.26 are available for development.

BaseLib/DynamicSpan.h

+/**
+ * \file
+ * \copyright
+ * Copyright (c) 2012-2021, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/project/license
+ */
+
+namespace BaseLib
+{
+template <typename T>
+struct DynamicSpan
+{
+    T* data;
+    std::size_t size;
+};
+}  // namespace BaseLib