Merge from trunk

git-svn-id: svn+ssh://scm.gforge.inria.fr/svnroot/gudhi/branches/add_utils_in_gudhi_v2@2732 636b058d-ea47-450e-bf9e-a15bfbe3eedb

Former-commit-id: 7872aa224ee471c05e9213b81192d304254e608a

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+File formats
+############
+
+Persistence Diagram
+*******************
+
+Such a file, whose extension is usually ``.pers``, contains a list of
+persistence intervals.
+
+Lines starting with ``#`` are ignored (comments).
+
+Other lines might contain 2, 3 or 4 values (the number of values on each line
+must be the same for all lines)::
+
+    [[field] dimension] birth death
+
+Here is a simple sample file::
+
+    # Persistence diagram example
+    2 2.7 3.7
+    2 9.6 14.
+    # Some comments
+    3 34.2 34.974
+    4 3. inf
+
+Other sample files can be found in the data/persistence_diagram folder.
+
+Such files can be generated with
+:meth:`gudhi.SimplexTree.write_persistence_diagram`, read with
+:meth:`gudhi.read_persistence_intervals_grouped_by_dimension`, or
+:meth:`gudhi.read_persistence_intervals_in_dimension` and displayed with
+:meth:`gudhi.plot_persistence_barcode` or
+:meth:`gudhi.plot_persistence_diagram`.