Add a section on scenario processing (#16)

Author: danielhuppmann

scenario-databases.rst

@@ -68,3 +68,51 @@ To select other (non-default) versions, you can use the "Switch to Advanced View
 in the scenario-selection tab of an IIASA Scenario Explorer or you can use the
 :code:`default_only=False` option of the function :func:`pyam.read_iiasa()`
 or the **ixmp4** package (`read the docs <https://docs.ece.iiasa.ac.at/ixmp4>`_).
+
+Scenario processing
+-------------------
+
+When submitting a scenario (a.k.a. "run") to an IIASA database instance, the server
+executes a scenario-processing workflow including *region-aggregation* and
+*scenario validation* prior to saving the scenario to the database. The processing uses
+the **nomenclature** package (`read the docs <https://nomenclature-iamc.readthedocs.io>`_).
+
+The region-aggregation and validation is configured via a project-specific GitHub_
+repository, usually named `https://github.com/iiasa/<project>-workflow`_. Please contact
+the respective project managers or the Scenario Services team if you need access.
+
+Executing scenario processing locally
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A project workflow can be run on a scenario locally, before submission to an IIASA
+database instance. This can be useful to test region-aggregation and validation.
+
+To run the project workflow locally, do the following:
+
+1. Install Python and run ``pip install nomenclature-iamc`` to install the package
+2. Git-clone the project repository
+   (usually `https://github.com/iiasa/<project>-workflow`_)
+
+Then, run the workflow on a scenario data file using the terminal (from the folder where
+the project repository was cloned to).
+
+.. code::
+
+    nomenclature validate-scenario <path-to-scenario-file>.xlsx
+
+The output log in the terminal will show any validation errors. If the output is empty,
+the validation was successful.
+
+.. warning::
+
+    Make sure to pull the latest project repository and update the **nomenclature** package
+    when you repeat this step later to make sure that the IIASA infrastructure and your
+    local check are in sync.
+
+Read the `User Guide`_ of the **nomenclature** package for more information!
+
+.. _GitHub: https://www.github.com
+
+.. _`https://github.com/iiasa/<project>-workflow`: https://github.com/iiasa
+
+.. _`User Guide`: https://nomenclature-iamc.readthedocs.io/en/stable/user_guide/local-usage.html