Setup simulation benchmarks

[jnnr]

This PR introduces some minimal examples for thermal networks and their simulation, together with the expected results data. This helps to

• define the problem that are supposed to be solved,
• define the data format in more detail.

In the end, there will be test cases that a simulation model implementation will need to fulfill.

Open TODOs:

• [x] Move the expected results and the two scripts generating it to /tests/_files
• [x] Write a test that runs the tree example and compares it to the expected results.
• [x] Delete the parts for single loop and single pipe- this will be filled with life later.
• [x] The scripts that reproduce the simulation benchmark results have to be adapted to conform with flake8's rules.
• [x] The test that compares the benchmark results with the results from dhnx has to pass.

[MaGering]

I've added the calculations for the example of the tree simulation. There are still some issues to discuss:

• The hight, which is needed for the calculation of the hydrostatic pressure difference, is missing as input value. For now I've chosen the latitude as a substitute, because the handling of its data appeared convenient to me. update Height is now included in the input data.
• The coefficients of local pressure losses are dependent on junction elements and devices. For this example I've made the assumption, that we have tee connectors at the inlet and return where the producer's and consumer's flows meet / split and valves at the consumers inlet as shown in the documentation's figure.
• I've made the calculations with a constant cp. If we drop this assumption we need to take it into account as we calculate the temperature of the mixture at the tee connectors.

[jnnr]

Thanks for your work! I corrected the path in the test and made it possible to compare data with lower precision.

Still missing: Columns and index differ between the default values and DHNx results.