Straightening FSC curves

The Fourier Shell Correlation (FSC) is traditionally calculated in resolution shells and plotted as a function of the inverse resolution. To ensure consistent statistics, these resolution shells should ideally contain an equal number of Fourier coefficients, at least approximately. This condition is naturally met when the shell boundaries are chosen uniformly in a scale based on the inverse cubic resolution. It has been observed that, in this scale, the FSC curves tend to exhibit local linearity. Using the new scale also zooms at the high-resolution shells allowing to extract values of interest more reliably.