In web development, there used to be several configuration files installed on each production server.
With the advent of cloud services, a persistent file system might not be available. Small micro-services
can be deployed and configured from pre-packed images (e.g. Docker) to scale up horizontally in short time.
These services usually come with some sort of base configuration, but credentials for related services
(e.g. a shared database) cannot be stored in the images. They must be provided at run-time through
environment variables.

So, in a production environment you usually want to configure your application through the environment,
but during devlopment it is quicker to share a default configuration through a file (e.g. `.env`).
This utility presents an easy way to read environment variables from three different sources.

This function allows to quickly access environment values and falls back to definitions in a local `.env` file.
This work is inspired by functionality common in web development.

It reads values from three sources in the following order:
1) System environment (getenv)
2) MATLAB preferences (getpref)
3) `.env` file on the path

Access to the `.env` file is cached, so subsequent calls do not impact performance as much.
The cache is invalidated whenever the `.env` file is modified on disk.