Implement restart for index files

As the index files might take some time, it is worth to implement a way to restart their generation.

doc/source/QuickStart/using_python.rst

@@ -23,7 +23,8 @@ To open a logfile, this object is call in a with statement:
 
 .. code-block:: python
 
-   with csds.Reader(filename, verbose=0, number_threads=1, number_index=10) as reader:
+   with csds.Reader(filename, verbose=0, number_threads=1, number_index=10,
+                    restart_init=False) as reader:
        # Use the Reader
 
 In this example, all the parameters are set with their default values.
@@ -34,6 +35,8 @@ regular interval of simulation time (e.g. every :math:`(t_e - t_b) / (N - 1)`
 where :math:`t_b` (:math:`t_e`) is the initial (final) time and :math:`N` the number of index file.
 To get the best performances, the best is to write an index file as close
 as possible and below any future requested time.
+As they might take some time to generate, it is possible to restart their generation
+from an interrupted run with the parameter ``restart_init``.
 
 Now the Reader can provide some information about the simulation: