Load execution ID from file

This MR introduces a new class IOHandler that encapsulates the most common I/O operations within the Theodolite framework. In order to do this, this MR also includes some restructuring/refactoring. With this handler, Theodolite uses a file expID.txt to store and load the execution ID for the next experiment.

slope-evaluator/README.md

@@ -8,7 +8,7 @@ For development:
 uvicorn main:app --reload
 ```
 
-Build the docker image:
+## Build the docker image:
 
 ```sh
 docker build . -t theodolite-evaluator