Improve guide on how to implement a model

These are proposed enhancements or bugs in the How to Implement Your Own Model in Utopia Guide.

Tasks

• Step one and two are written for non group members. Where the folder structure is src/utopis/models and then there is a note that CCEES members should go tho the models folder, but there the folder structure is src/models. This is confusing.
  • My recommendation is to write the guide suitable vor CCEES members, since we are the ones using utopia and give notes to non group members.
  • Also is would be nice to include the instructions on how to get the Models respository into the how to build a model guide.
  • Problems occurred in step 10-12, when running the first test and getting confused with where to place the build directory and where to execute cmake and make.
• Formerly #121 (closed): In step 13, one is instructed to perform plots, but they don't work because they haven't been set up yet. So either make sure the plots work or add --no-plot there.
• Formerly #122 (closed): In the plotting-related section, the generic.py file seems not to exist.
  • Also, it seems to be confusing where the *_plots.yml file is located; improve that sentence.