Pytzer

* Rewrote central excess Gibbs energy and equilibrium solver functions so they are differentiable with respect to temperature and pressure.
* Structure of parameter libraries revised and mechanism for switching between them updated.

* Created new `Humphreys22` parameter library and validated its calculations against [HWT22](../refs/h).
* Created new `Clegg22` parameter library (but its calculations against [CHW22](../refs/c) still need validating).
* Created new `Clegg23` parameter library and validated its calculations against [CWTD23](../refs/c).
* Renamed `io` module as `get`.

* Fixed calculation error in `convert.solvent_to_solution`.
* Added more ions in `properties.ion_to_mass.`

* Fixed `pz.convert.solvent_to_solution` to take and return p*K* values instead of *K* values, and to replace any NaNs in the input with 0.
* Added `pz.equilibrate.stoichiometric.get_constants()` to calculate stoichiometric equilibrium constants for the carbonic acid equilibria when total dissolved inorganic carbon is zero.

* Switched from Autograd to [JAX](https://jax.readthedocs.io/en/latest/) for faster automatic differentiation and JIT compilation.
* Renamed and reorganised virtually everything.

* Added `osm` function for osmotic coefficient to **matrix** module;
* Added **io** module functions to roughly convert between molinity (mol/kg-solution) and molality (mol/kg-H₂O);
* Added `salinity2mols` function in **io** module to estimate molalities from salinity for [MZF93](../references/MZF93) artificial seawater;
* Switched `assemble` function in **matrix** module to use the MarChemSpec parameter library by default.