Skip 01_tutorial_getting_started if graphviz is not installed

Cell 19 of Tutorial 1 requires graphviz. Skip if not installed.

Alternative would be to execute this only if graphviz is installed.

conftest.py

@@ -85,6 +85,7 @@ try:
     import graphviz
 except ImportError:
     collect_ignore += [os.path.join(SCRIPT_FOLDER, "pystencils/backends/dot.py")]
+    collect_ignore += [os.path.join(SCRIPT_FOLDER, "doc/notebooks/01_tutorial_getting_started.ipynb")]
 
 try:
     import pyevtk