create and populate an examples directory

[bch0w]

Although SeisFlows has some 2D examples, the method for running them is slightly opaque as they're hidden behind command line functions, and explained in the online documentation.

I think similar to SPECFEM, SeisFlows should also have an examples directory with some populated subdirectories that contain examples featuring SPECFEM2D, 3D, and 3D_GLOBE, and different workflows (forward, migration, inversion etc.). Some relevant waveform data may be available for comparison, or appropriate links to online data storage.

This would help new users move past the more simple 2D examples, act as a storage repository for published studies which make use of SeisFlows, and serve as a set of integration tests.