Add inactive sensors to graph

Created by: RasmusOrsoe

This PR is a draft. It's a bit rough around the edges but outlines a method of adding inactive sensor modules to the graphs.

The general idea is to add abstract (backend specific implementations needed) methods to the Dataset class that queries geometry_tables for sensor_id's. In SQLite I've created a method that constructs geometry_table (look-up table for geometry) from pulsemap tables.

Right now, the _get_inactive_sensors will actually extract the active pulsemap on its own (and create the UID in the process). This means that every pulsemap will be extracted twice; a bit inefficient.

Let me know what you think of the general approach! I'll try to polish this up during the week.