set quadrature degree for forms in equation residual

[jshipton]

This PR ensures that we no longer unthinkingly exceed sensible quadrature degree limits. This is achieved by adding a label_map to the equation residual in the setup_equation method of the Timestepper class which replaces all of the terms with new terms containing the same form and labels, the only difference being that the integrals in the form have had their metadata updated to contain a quadrature degree of 5.

To Do:

• use logging to log an info level warning when the quadrature degree is set
• add max_quad_deg to other equation sets
• use max_quad_deg in initialisation (and diagnostics?)