Some refactoring of DMUser & DMUserMultiBehavior

• Do the log from log_user_stat.hpp in DMUser rather than in DMUserMultiBehavior
• test the log for DMUser tests -> see issue #25
• add class DMUserDegradTemp / rename DMUserDegrad -> DMUserDegradSpatial + make DMUserMultiBehavior inherit from DMUserDegradTemp
• same for class SeeULater
• fill in the file src/users/user_description_file_template.json for documentation