Add a new format file for surface source

docs/source/io_formats/index.rst

@@ -44,6 +44,7 @@ Output Files
 
    statepoint
    source
+   surface_source
    summary
    depletion_results
    particle_restart

docs/source/io_formats/surface_source.rst

@@ -0,0 +1,21 @@
+.. _io_surface_source:
+
+==========================
+Surface Source File Format
+==========================
+
+When surface source writing is triggered, a separate source file is written with
+only the sources on specified surfaces. The format is identical to source file.
+
+**/**
+
+:Attributes: - **filetype** (*char[]*) -- String indicating the type of file.
+
+:Datasets:
+
+           - **source_bank** (Compound type) -- Source bank information for each
+             particle. The compound type has fields ``r``, ``u``, ``E``,
+             ``wgt``, ``delayed_group``, ``surf_id`` and ``particle``,
+             which represent the position, direction, energy, weight,
+             delayed group, surface ID, and particle type (0=neutron, 1=photon,
+             2=electron, 3=positron), respectively.