Update readme with python example

README.md

@@ -27,12 +27,50 @@ pip install python-unsio
 ```
 ## Usage
 
+- example : load gas and stars component of an UNS compliant snapshot
 ```python
-import unsio.input as unsi
+import unsio.input as uns_in
 
 # we instantiate object
-x=unsi.CUNS_IN()
+myfile="snashot.000"
+my_in=uns_in.CUNS_IN(myfile,"gas,stars")
+
+# load snapshot
+if my_in.nextFrame():
+  # read stars positions
+  status,poss=my_in.getData("stars","pos")
+  # read gas positions
+  status,posg=my_in.getData("gas","pos")
+  # read gas densities
+  status,rho=my_in.getData("gas","rho")
+  # read time
+  status,timex=my_in.getData("time")
+
 ```
+- example : save previously loaded data in gadget3 format
+```python
+import unsio.output as uns_out
+
+# we instantiate object
+myoutfile="snapshot.g3"
+my_out=uns_out.CUNS_OUT(myoutfile,"gadget3")
+
+# prepare data to be saved
+# set time
+status=my_out.setData(timex,"time")
+# set positions for stars
+status=my_out.setData(poss,"stars","pos")
+# set positions for gas
+status=my_out.setData(posg,"gas","pos")
+# set density for gas
+status=my_out.setData(rho,"gas","rho")
+
+# save on disk
+my_out.save()
+# close
+my_out.close()
+```
+
 ## License
 UNSIO is open source and released under the terms of the [CeCILL2 Licence](http://www.cecill.info/licences/Licence_CeCILL_V2-en.html)