Adjust error analysis.

Previous test was placing a box centered at each of the two test charges, so wasn't actually testing the order calculator well. Use 3 test charges instead to avoid this.
Make fastguasstransform generic over eltype.

Switch tests to test Float32 and Float16 data in addition to Float64.
Switch to O(1) search for relevant centers in evaluate.
Fix type stability.

Add a simple benchmark.

Found that extra keyword args to fastgausstransform made it type unstable. They
were mostly for scaffolding purposes anyway--all a user should care about is
rtol--so just remove them.