Hybrid algebra/ode solver

[teddygroves]

This change implements an idea we unsuccessfully tried a while ago, namely finding steady states using an algebra solver with an embedded ode solver as described here. It's now way easier than before thanks to the variadic algebra solver.

Sort of addresses #368: it's a hybrid solver but with the bdf solver not the adjoint solver.

Note that after this change there are two configuration options for solvers: ode_solver_config and algebra_solver_config. They are specified like this in config.toml:

[ode_solver_config]
rel_tol=1e-12

[algebra_solver_config]
abs_tol=1e-8
max_num_steps=1e5

Checklist:

• Updated any relevant documentation
• Add an adr doc if appropriate
• Include links to any relevant issues in the description
• Unit tests passing
• Integration tests passing

[NicholasCowie]

Works on most models tested