Nothing new, just improving versioning

No features. This just reduces the number of places version numbers are used from 3 (setup.cfg, docs/conf.py, laserbeamsize/__init__.py) down to just the __init__.py file.

v1.9.1

• centralize version number to a single place

v1.9.0

• add beam_ellipticity()
• add beam_parameter_product()
• rotate x-tick labels when more than 10 ticks
• removed deprecated use of np.matrix()
• M2_report now includes BPP values
• improve API docs
• code linting