Output time from a file

@matthieu, If you wish to already review feel free, I will only modify cosmology_get_scale_factor in order to read from a table.

Everything else is implemented and ready to use.

Test I have done:

• Run SedovBlast_3D with output_list for snapshots and stats and checked the output times.
• Run partially SedovBlast_3D and restarted.
• Run CoolingBox with cosmology and checked the transformation redshift->scale_factor and time->scale_factor.

doc/RTD/source/GettingStarted/parameter_file.rst

@@ -12,6 +12,35 @@ parameters.  Each section in this file corresponds to a different option in
 SWIFT and are not always required depending on the configuration options and
 the run time parameters.
 
+Output List
+~~~~~~~~~~~
+
+In the sections ``Snapshots`` and ``Statistics``, you can specify the options ``output_list_on`` and ``output_list``  which receive an int and a filename.
+The ``output_list_on`` enable or not the output list and ``output_list`` is the filename containing the output times.
+With the file header, you can choose between writing redshifts, scale factors or times.
+
+Example of file containing with times (in internal units):
+::
+   # Time
+   0.5
+   1.5
+   3.0
+   12.5
+
+Example of file with scale factors:
+::
+   # Scale Factor
+   0.1
+   0.2
+   0.3
+
+Example of file with redshift: 
+::
+   # Redshift
+   20
+   15
+   10
+   5
 
 Output Selection
 ~~~~~~~~~~~~~~~~