Field data types must be enforced consistently

I'm trying to debug the line searching algorithm and noticed that some of the input quantities have complex type.

This is not really acceptable, because there are no useful comparison operators defined for complex numbers, and the code does lots of comparisons. I don't understand why no exceptions are thrown; pure Python immediately complains if complex numbers are compared via <,>, etc., but numpy somehow doesn't.

We really need strict enforcement that search positions, gradients and energy values have real type.