Features/plot scalar resources

[jnnr]

This introduces a script and a snakemake rule for plotting scalar resources.

[jnnr]

Thanks for your work! This is almost finished. Just a few steps cleaning up to help our future selves.

[jnnr]

Hi @juliusmeier: I made some adjustments to keep up with the latest developments and show the script in the docs. To finish this PR, please:

• Decide: Is the script generic or specific, i.e. can it be used to plot other resources?
• Use load_b3_scalars.
• Make sure the preprocessing of conv_pp uses the data processing functions of oemof-B3. Related issue: Include custom process data functions in data processing of conv power plants #58
• Sort imports by running isort

[juliusmeier]

* Decide: Is the script generic or specific, i.e. can it be used to plot other resources?

Due to the pivot table which is needed to bring the data in the necessary format to make a grouped bar plot of the conv pp capacities in each region, the script is specific.