I'm not using the ideal estimate of the integral length since I'm doing
the integral with the 1D spectrum, but I think this should be close
enough.

in practice nothing changes, but the spectra will be shorter and the kM
eta calculation will be "correct".

it's now based on the last git commit

rFFTW_interpolator->descriptor is not allocated in the constructor,
therefore it should not be deallocated in the destructor

it's not more efficient.

I don't really like it this way, but otherwise it won't compile when
needed.

somewhat.
Python 3 insists on generating UTF-8 datasets with h5py, unlike python
2. I guess this commit deals with the problem.

io fails at the moment

I don't like the way that I'm doing it now, but I don't like the old way
more. With a static library, I no longer need to worry about installing
the package while running a simulation, etc.
Also, now I have full control over the arguments.