[BUG] interface constraints for PLI don't work

[lachlangrose]

trafficstars

Describe the bug Interface constraints are implemented incorrectly and produce weird solutions. The interface constraint should try to minimise the difference in the interpolated scalar field value for all pairs of points with the same line.

In FDI this is done using a for loop which is slow, PLI has an attempt to do this using NumPy but does not work.