Clean-up of the linear SDE example

[divijghose]

This PR adds functionality to make the linear SDE more user-friendly and cleans up the example file:

1. Instead of hard-coded values, the configuration (Final time, number of steps, SDE parameters, particles per rank, etc.) are now read from either a YAML configuration file or from command line arguments.
2. All codes have been formatted using black for consistency.
3. Import errors, possibly arising from a new Firedrake version, have been fixed.
4. gitignore has been updated for a cleaner repository.