because that's what it is...

acc errors seem a bit off though

1 particle species seems to be correctly integrated at the moment.

this way, rFFTW_interpolator no longer needs to include interpolator.
which will help with splitting the library.

also, tests are no longer executable files. I guess I no longer need
that particular hack...

also, put in a more reasonable default value... although it doesn't seem
to be that reasonable in terms of actual results.