Mud-examples

extras` is now `datasets` and some new pde-related experiment orchestration code has been added.

argument swaps, plus poisson plotting

Code to generate the linear examples is migrated, with the exception of plotting functionality.

adding helper method to iterate over measurement quality changing.

convenience methods I wrote for the nonlinear examples in the paper. These basically take a wrapper for a mud problem and run experiments with increasing numbers of measurements, then generate the summary statistics for the errors.