Improving flow rate calculation

[daklauss]

Optimized the calculation of the Flowrates in get_flow_rates( )of every unit operations by creating a matrix as described in the Forum and solving it by using numpy.linalg.solve.
Rewrote pretty much the whole function including generation of the return dict.

The former solve_flow_rates( ) got removed because it became unnecessary.

Also added a new testcase i used for implementing the new calculation method.

I was a little bit to greedy with black and pylint ....

PS. i added .vscode to .gitignore because my vscode created some weird files for testing i didn't want to comit

TODOS:

• Review documentation
• Cleanup commits @schmoelder
• Check case where unit is only connected to itself (undefined solution if no given q_out)
  • Add check/raise exception
  • Add testcase

[schmoelder]

Thanks @daklauss for this PR! Code looks good, just some minor formatting points. I'm looking forward to seeing some benchmarking results!