DOC: Add user guide sections

[inoelloc]

Currently, the user guide is very short and only gives you a quick introduction to smash. Other sections need to be added to the user guide. There is already an issue about bayesian estimation case study #128

[pag13]

I agree, before reaching a didactical and more complete user guide, few ideas of tutorials with existing features:

• tuning of the complexity of the regionalization ANN ; others
• multi gauge calibration
• use of camels or other data bases
• simulation with continuous evapo ?
• ...

[pag13]

Also low complexity tutorials for beginners (not a hydrology course but simple practice associated to..)