Update IPT feature branch with code used in study

[JosephThomasParker]

This PR pushes the code that was actually used in our manuscript on the UKAEA pinboard into the feature branch.

The changes relative to next+ipt are:

• Use a single PVODE-like error check, error = residual/(rtol*soln + atol) < 1 instead of separate checks on rtol and atol.
• Do a convergence check before doing any work. This saves work when calling this solver inside an iterative time advance, in which case we can sometimes start in a "converged" state.
• Revert a function encapsulation that caused segfaults (I didn't get a chance to properly investigate this).