Epic: Implement RELM 5-year forecast test

[DanijelSchorlemmer]

For the data structure and the interaction for running long-term CSEP tests (e.g. the RELM 5-year experiment), we need to implement:

• Database classes (AbstractDatabase, SpatiaLiteDatabase) (Implementation of basic database classes #4)
• Abstract CSEP class and SpatiaLiteCSEPDatabase class (Implement the base CSEP class and the derived class CSEP class for SpatiaLite databases #6)
• Catalog handling
  • Table definitions (Catalogs, Events)
  • Interface functions
  • Catalog import
• Grid handling
  • Table definitions (Grids, Cells, Bins)
  • Interface functions
  • Grid import
• Model handling
  • Table definitions (Models, Forecasts, ForecastRates)
  • Interface functions
  • Model import
• Implement the access to the RELM forecasts from Zenodo
• Create example code to run the 5-year RELM test

With this implementation, the example codes showing the 5-year RELM test can later be used as acceptance tests for the dbCSEP package.