Data items in the EM_MAP_RESOLUTION category record details
about the Fourier Shell Correlation
The most popular method at the moment for resolution assessment is the
Fourier Shell Correlation, although there is no standard way to define
the cutoff value. It assumes a uniform distribution of the resolution
along the three-dimensions (so there is a single value of resolution).
The Fourier Shell Correlation curve is defined as a list of value pairs
(x, y) where x is the spatial frequency (or inverse resolution in 1/A)
and y is the correlation coefficient (a value between 0 and 1).