Implement: pip package, providing CLI tool

[apasto]

From the concept-stage script to a package, with the necessary refactoring, and the configuration for setuptools.

Summary:

• old linreg-grids.py -> modules in decorrelategrids package
• refactor to separate the regression functions, the "cli wrapper" (argparse etc.), and the actual cli tool
• setup.py with metadata, including entry point to install the cli tool
• get rid of deps environment.yml (moved to setup), except for pip

See also: entry in roadmap #2