Add number_of_moles property and extensive thermodynamic properties to Material

[bobmyhill]

BREAKING CHANGES

This pull request refactors the handling of extensive and molar properties, primarily by standardizing the use of number_of_moles and providing clear separation between extensive and molar quantities.

Major changes:

• Added number_of_moles property to Material, with getter and setter, and refactored all relevant classes to use this standardized attribute. By default, number_of_moles is equal to 1 for all classes.
• Removed use of the undocumented, invisible n_moles attribute.
• Introduced extensive property getters (e.g., internal_energy, volume, entropy, etc.) in the parent Material class, which all multiply molar properties by number_of_moles.
• Updated aliases (e.g., V, S, H, C_v, C_p) to reference extensive properties instead of molar ones. This is the only major user-facing break in behaviour
• Refactored composite property calculations (phase_volumes, phase_masses, formula) to use extensive properties
• Updated check_eos_consistency() in burnman/tools/eos.py to work directly with extensive properties

Separately, this PR also adds cleaner error messaging and try/except blocks for when properties are requested before set_state() has been called.

[bobmyhill]

Addresses #707