Add rule to clean workflow output

[ekatef]

Closes #14

Changes proposed in this Pull Request

Added a simple rule to remove output of the workflow which may be useful to avoid messing-up data for different configurations.

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [ ] Code and workflow changes are sufficiently documented.
• [ ] Newly introduced dependencies are added to the main environment at PyPSA-Earth repository
• [ ] Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• [ ] Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• [ ] Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• [ ] A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[ekatef]

I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0 ;) Probably, it makes sense to add this to a PR template

[ekatef]

Currently, clean rule removes resources/demand/microgrid_load.csvwhich is actually tracked by git, but is also calculated by build_demand. Not sure if that is a desirable behaviour for a cleaning rule, and happy to find workaround.