DOC: Correct conda usage in user guide

[inoelloc]

There is still note about using conda in the user guide. Since the version 1.0, you no longer need to use conda to run smash

[asjeb]

Add a section about creating conda environment which contains pip :

• conda install pip
• pip install smash-hydro
  Motivations :
• Manage conflicts with the system
• Pip doesn 't work directly on the server