Logger restart

This should not be merged before !813 (merged) and needs to be redirected towards master.

Here is the implementation of the restart. This is very very simple.

examples/HydroTests/SedovBlast_3D/sedov.yml

@@ -34,3 +34,12 @@ InitialConditions:
   file_name:                    ./sedov.hdf5          
   periodic:                     1
   smoothing_length_scaling:     3.33
+
+# Parameters governing the logger snapshot system
+Logger:
+  delta_step:           10     # Update the particle log every this many updates
+  basename:             index  # Common part of the filenames
+  initial_buffer_size:  0.01      # (Optional) Buffer size in GB
+  buffer_scale:	        10     # (Optional) When buffer size is too small, update it with required memory times buffer_scale
+  time_first_index:     0.    # Time of the first output (in internal units)
+  delta_time_index:     1e-2  # Time difference between consecutive outputs (in internal units)