Add comprehensive documentation for units feature

[Copilot]

Addresses issue #242 by adding comprehensive documentation for the units feature that was implemented in PR #239.

Overview

The mendeleev package now supports accessing physical properties with proper units using the Pint library. Users can append _u to any property name to get a Quantity object with units attached, enabling safer calculations and easy unit conversions.

Example Usage

from mendeleev import H, Al
from mendeleev.models import ureg

# Access properties with units
print(H.atomic_weight_u)  # 1.008 dalton
print(Al.density_u)       # 2.7 gram / centimeter ** 3

# Unit conversions
temp = Al.melting_point_u
print(temp.to('celsius'))  # 660.323 degree_Celsius

# Safe calculations
volume = 1 * ureg.liter
mass = Al.density_u * volume
print(mass.to('gram'))     # 2700.0 gram

Documentation Added

1. Comprehensive Reference (docs/source/data_access.rst)

• Complete listing of all 80 properties across classes that support units
• 63 Element properties organized by category (atomic, thermal, electronic, etc.)
• Coverage of IonicRadius, IonizationEnergy, Isotope, PhaseTransition, and ScatteringFactor classes
• Detailed examples and error handling documentation

2. Interactive Tutorial (docs/source/notebooks/working_with_units.ipynb)

• Step-by-step tutorial with practical examples
• Unit conversions, calculations, and multi-element comparisons
• Error handling and edge cases
• Integration with other model classes

3. Quick Reference (docs/source/quick.rst)

• Brief introduction to units feature for discoverability
• Simple example to get users started

Properties with Units Support

The documentation covers all supported properties including:

Element properties: atomic_weight, density, melting_point, boiling_point, atomic_radius, covalent_radius_, vdw_radius_, abundance_*, electron_affinity, dipole_polarizability, thermal_conductivity, and many more.

Other classes: IonicRadius (ionic_radius, crystal_radius, charge), IonizationEnergy (ionization_energy, uncertainty), Isotope (mass, quadrupole_moment), and others.

Testing

All documented examples have been thoroughly tested to ensure they work correctly, including:

• Basic property access with units
• Unit conversions using Pint
• Calculations preserving units
• Error handling for properties without units
• None value handling for missing data
• Cross-class functionality

This completes the TODO item from PR #239 and provides users with comprehensive guidance on using the units feature.

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