FFTW and copy construction

[cglosser]

Somehow, destroying an object with a TransformPair member seems to cause a segmentation fault (looks like the TransformPair destructor tries to free something it can't). This makes it problematic to copy AIM::Farfield objects or return them from functions, so it should probably be fixed.

[cglosser]

In particular this is why the make_interaction() method in field_tests.cpp returns unique pointers.