Improve input loading
Created by: guillaume-plc
Split the input loading methods into smaller generic methods and extend their functionality in order to:
- Handle categorical input data (= data that cannot be interpolated row by row, such as evolving substitue groups) Categorical data tables must contain "category" in their table name to be properly handled.
- Handle data without a year and/or value column in the scenario database: concatenate static and scenario data (overwriting static with scenario in case of overlap)