Gadopt

What's Changed
* Allow for more generic configuration of plate reconstruction datasets by sghelichkhani in https://github.com/g-adopt/g-adopt/pull/109
* Add DG viscoplastic case by drhodrid in https://github.com/g-adopt/g-adopt/pull/115
* Add free surface code by w3168 in https://github.com/g-adopt/g-adopt/pull/51
* PDE-constrained optimisation by drhodrid in https://github.com/g-adopt/g-adopt/pull/129
* Viscoelastic with stress defined in approximation by w3168 in https://github.com/g-adopt/g-adopt/pull/122

New Contributors
* dsroberts made their first contribution in https://github.com/g-adopt/g-adopt/pull/131

**Full Changelog**: https://github.com/g-adopt/g-adopt/compare/v3.0.0...v3.1.0

What's Changed
* Reorganise test/demo locations
* Allow assigning 1D profiles from files to functions
* Add min/max temperature diagnostics
* A fix for vertical_component and upward_normal in 3d extruded meshes
* Add pyGplates interface
* Make `cartesian` a mesh attribute
* Allow for variable gravity
* Pressure nullspace

**Full Changelog**: https://github.com/g-adopt/g-adopt/compare/v2.3.0...v3.0.0

This release rewrites many of the demos in a jupytext-compatible notebook format. As a result, the `testing` optional dependency set has now become `demos`.

An API-breaking change is that the `GeodynamicalDiagnostics` object now accepts a mixed function, rather than velocity and pressure separately.

This release updates the structure of the G-ADOPT package and its dependencies, making it easier to install.

This is the candidate release of the new G-ADOPT library. This handles a lot of the previously-duplicated code at the library level, including automatic handling of solver parameters, many timestepping schemes, and physical approximations. The library is regression-tested to be compatible with the previous release. It includes further improvements to the codebase and some solve parameters consistent with recent PETSc & Firedrake changes.

This is the candidate release of the new G-ADOPT library. This handles a lot of the previously-duplicated code at the library level, including automatic handling of solver parameters, many timestepping schemes, and physical approximations. The library is regression-tested to be compatible with the previous release.