Adds spatial frequency map functions
Adds spatial frequency map functions

this adds two new functions: `create_sf_maps_cpd` and
`create_sf_maps_cpp`, which return maps of the spatial frequency in
cycles per degree and cycles per pixel, respectively. this is useful
because it's often what we want to reason about directly

billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences