Provide conventional FWI convenience function

[ar4]

Although it is not difficult to write a simple FWI script using Deepwave, some casual users may prefer to have a function that they simply call to run FWI for them. It could also eventually include some popular variants of FWI, such as inverting from low to high frequencies, source inversion, phase-only inversion, and commonly-used regularization methods, etc.

The function should probably have as its required arguments the true receiver data amplitudes and locations, source locations, dt, the migration aperture, and perhaps topology. This should be enough to create a sensible initial model and source amplitudes, and begin inversion.