add doxygen file comment.

src/external/nexus/PNeXus.cpp

@@ -27,6 +27,46 @@
  *   59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.             *
  ***************************************************************************/
 
+/**
+ * @file PNeXus.cpp
+ * @brief Implementation of the PNeXus class - NeXus HDF4/HDF5 file reader and writer
+ *
+ * This file contains the implementation of the PNeXus class for reading and
+ * writing ISIS muon NeXus HDF5 files. It provides case-insensitive path
+ * lookup functionality using the HDF4 C / HDF5 C++ API.
+ *
+ * The implementation includes:
+ *
+ * **Read Functionality:**
+ * - String utilities for case-insensitive comparison
+ * - Path parsing for HDF4 / HDF5 hierarchical paths
+ * - Recursive path traversal with case-insensitive matching
+ * - Attribute, group, and dataset lookup functions
+ * - Support for IDF version 1 and 2
+ *
+ * **Write Functionality:**
+ * - Group hierarchy creation
+ * - Dataset writing for int, float, and string types
+ * - Attribute preservation
+ * - Multi-dimensional array support
+ * - Type-safe operations using PNXdata template class
+ *
+ * **Error Handling:**
+ * - Comprehensive error handling with descriptive exceptions
+ * - Validation of data consistency
+ * - HDF4 error propagation / HDF5 exception propagation
+ *
+ * @author Andreas Suter
+ * @date 2007-2026
+ * @copyright GNU General Public License v2
+ * @version 1.0
+ *
+ * @see nxH4::PNeXus
+ * @see nxH4::PNXdata
+ * @see nxH5::PNeXus
+ * @see nxH5::PNXdata
+ */
+
 #include <iostream>
 #include <fstream>
 #include <sstream>