v0.3.0

This is the third release of the package, and has some excellent updates!!!

1. Now the package uses poetry as the package manager and no longer uses setup tools for installation. From now on we can use a more "modern" python packaging system with pep 517. It also helps to resolve the dependency conflict.
2. Now the package supports any planets other than Earth by providing r_planet in SeisModel and planet_radius in Config. Note we have to provide them all at the moment, future versions might directly derive planet_radius from r_planet. The default values for them are 6371.
3. Now the noise suppression factor sigma has can be adjusted in Config (default to 2). It is hard to describe how to adjust this value, but the suggestion is that to use 2 first, and adjust it if there is too much numerical noise.

Full Changelog: v0.2.0...v0.3.0