Fix/interppoints_3DH1D

Fix for interppoints used in 2.5D cases. Main changes:

• Modified ExpList3DHomogeneous1D constructor to store graph2D.
• Fixed interpolation in the z-direction by making interpolator look for correct plane, resetting to closest plane if the point lies outside of it.
• Added regression 4 tests with a spanwise-varying field, testing plane and box in serial and parallel.
• Modified user guide to reflect code changes.