GPU-accelerated DFL viscoacoustic wave equation modeling

It is a python package for DFL viscoacoustic wave equation modeling based on GPU parallelization.
  The package provides balances between accuracy and efficiency:
    for accuracy: it provides two-step extrapolation scheme with time-stepping error compensation (2nd-order $k$-space compensators and their simplified version);
    for efficiency: the package is utilizing CuPy library to perform 3-D FFTs and elementwise computations; it also provides more efficient natural-attenuation ABCs (naABCs) besides the conventional hybrid ABCs.

The naABC-related functions and classes are separately documented in "gvopt.py", which is readily applicable in other viscoacoustic modeling schemes.