Design problems should have a default init_soc

[NicolaCourtier]

Python Version

3.10

Describe the bug

If an init_soc value is not passed to DesignProblem, then predictions may start from incompatible initial states. For example, the formation concentrations may be taken as (infeasible) initial concentrations, which returns the following error message. To resolve this issue, init_soc should either be a required input or take a default initial value for design optimisation problems.

Steps to reproduce the behaviour

Check by removing the init_soc parameter from the spme_max_energy example script.

Relevant log output

pybamm.expression_tree.exceptions.SolverError: Step 'Discharge at 1C until 2.5 V (5 seconds period)' is infeasible due to exceeded bounds at initial conditions.