Rpgpy

- Level 1 data support to `rpg2nc`
- Bug fixes

This release:

- Fixes bug in datestamp to date conversion
- Adds support for Level 1 V4 data
- Adds support for Python 3.8
- Adds first unit and end-to-end tests and Github Actions CI
- Fixes installation instructions

This version fixes spectral matrices so that the difference chirps correspond to each other.

Version `0.3.1.` adds bug fixes.

First stable release.