Improve Beam docstrings and type hints

[Hespe]

Description

The outdated docstrings and properties are adjusted to match the new naming scheme.
In addition, Beam is converted into an abstract class, increasing the probability to catch such issues early.

(The old mu_s property of Beam is renamed to mu_tau.)

Motivation and Context

The beam coordinate system has been adjusted recently, leaving some outdated docstrings in the Beam class and its subclasses ParameterBeam and ParticleBeam.

• I have raised an issue to propose this change
  • Misleading docstring and type hints in beam class methods #238
  • Outdated property in Beam class #279

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

Note: We are using a maximum length of 88 characters per line.

[Hespe]

I had planned on changing from_parameters() and from_twiss() to use the same default number of particles for ParticleBeam. However, some tests rely on the old behaviour, so I reverted that change for now.