Update and fixate requirements

[Haifischbecken]

Describe the bug
Installing the Pyvolt package in an up-to-date environment and executing one of the example scripts leads to an error.
This is caused by changes in Numpy that has removed support for numpy internal datatypes (np.int, np.complex etc.) and replaced them with Python default datatypes.

To avoid this problem in the future Versions should be pinned.
While at it the project could/should be transitioned to the currents standard of PEP621 for project metadata (see e. g. Setuptools explaination

To Reproduce
Steps to reproduce the behavior:

1. Pull repo and cd into it
2. (create venv)
3. pip install .
4. python examples/quickstart/run_nv_powerflow.py