Good to have source terms

[timofeymukha]

Once we merge #962 we can add various source terms for users to enjoy. Which ones are good to have, let's list them here. It could be a relatively easy task to pick one up for someone wanting to contribute something compact to the code. But let's not think about who will do it here, but rather just what would be nice. I provide some initial points.

• [ ] Coriolis force.
• [ ] Rotating reference frame.
• [ ] Tripping line.
• [ ] Sponge region.
• [ ] Boussinesq approximation.
• [ ] Actuators, line/disk.
• [ ] Solution filtering.