Reorganize project following standard Python conventions

[larsks]

This commit relocates all of the Python source files into the batchtools
package. This, along with appropriate modifications to pyproject.toml,
mean that:

• We now have an installable package.

  From within this directory, you can run pip install . to install the
  package on the local system.

  You can also install the package straight from the GitHub repository:

  pip install git+https://github.com/memalhot/python-batchtools
  

• We install a runnable batchtools script.

  By setting project.scripts in pyproject.toml, we install a
  batchtools command when we install the package. This permits you to run
  the code by simply typing batchtools at the command line, just like any
  other command.

  You can run the script without installing it using the uv run command:

  uv run batchtools
  

• We have a distributable package.

  Running uv build will produce a distributable Python package (both
  a .tar.gz file and a Python wheel 1) in the dist/ directory. This
  can be uploaded to PyPi using uv publish, or simply copied and
  installed somewhere using pip.