Adds scalar double backpropagation

Also fixes bug in elastic PML that caused suboptimal absorption

Change distribution to precompiled libraries

Windows, pressure and background receivers

* Minor code changes to allow compiling on Windows
* Pressure receivers in the elastic propagator
* Background wavefield receivers in scalar_born

Minor bugfix for PyTorch v1.9

Adds elastic propagator

Improve PML

This commit changes the PML profiles ("a" and "b") resulting in
better absorption

Hicks, freq_taper_frac, and time_pad_frac

This release introduces three new features:
    * Interpolation of arbitrary locations onto the grid using the
      Hicks method
    * Frequency tapering to reduce ringing
    * Time padding to reduce wraparound artifacts

It also adds new examples, and fixes a bug that could cause the PML
to become unstable.

Substantial performance improvement

Minor fix for older versions of nvcc

Compiling failed with version 10.1 of the nvcc compiler (although was
successful with version 11). This fix removes the problem that caused
the older compiler to fail. This issue was reported in #44.

v0.0.11

This is a minor bugfix release. The most important change is
to fix a bug that resulted in an incorrect gradient at source
locations in scalar_born when the timestep interval was greater
than the CFL limit.