Implement and run doctests #241
Description
Now that we have examples in the documentation (#208), we can run at least some of them as doctests, mostly to make sure that the examples continue working as intended. Most of the examples involve applying some topotoolbox function and plotting the result, so the tests may not be very informative, but they can at least alert us to internal or external API changes that we may not have caught, and they can help us increase our test coverage.