A complete rewrite. 1D and 3D propagators have been removed to focus
on the more popular 2D. Among the new features are a switch to a more
accurate PML, the addition of the ability for users to choose the
finite difference order of accuracy, to provide initial
wavefields, to calculate the gradient with respect to the wavespeed
and source amplitudes in the Born propagator, and to apply PyTorch's
JIT to the modules. The final wavefields are now included in the
outputs of both the regular and Born propagators, and can be used
when calculating gradients.