* Adds a scalar wave equation Born propagator, enabling demigration and
LSRTM.
* The source amplitude (including the receiver gradient during
backpropagation) is now interpolated using scipy.signal.resample, rather
than using nearest neighbour interpolation.
* The C++ and CUDA code has been reformatted to use Google's code style,
and (hopefully) made easier to understand by replacing the macro
function with regular functions.