Discuss changing options_.hp_ngrid

The option options_.hp_ngrid is now badly named after introducing a bandpass_filter option. It actually governs the number of points used in the inverse Fourier transform for any filter (see #1011 (closed)). I would suggest creating a new option ifft_points in the options structure and the preprocessor. For backward compatibility we should keep hp_ngrid in the preprocesor, but have it now map into options_.ifft_points