Release v0.4.0

[raphaeltimbo]

Bumps the version to 0.4.0 and adds release notes to the docs (new Release Notes section on Read the Docs).

The notes cover everything merged since v0.3.32: #136, #137, #138, #140, #141, #142, #143, #144, #145, #146, #147, #148, #150, #151, #152.

Highlights:

• Constraint-propagation point solver replacing the _calc_from_* dispatch (Replace Point _calc_from_* dispatch with a constraint-propagation solver #138)
• gp_surrogate conversion method on Impeller.convert_from (Add gp_surrogate method to Impeller.convert_from #144)
• JSON/TOML save/load with public to_dict/from_dict (Save class objects in json and toml #145, Refactor file serialization: suffix-driven formats, public to_dict/from_dict #148)
• Dense-fluid conversion and REFPROP concurrency fixes (Fix convert_from for dense fluids: robust isentropic discharge and efficiency solve #141, Fix REFPROP concurrency: GIL-holding DLL load, coherent fallbacks, non-fork pools #150)
• CoolProp 8 compatibility (Add CoolProp 8 compatibility, drop coolprop<8 dev constraint #152)

After merge, tag v0.4.0 on the merge commit to publish the release.