size must be a scalar
size must be a scalar

with this commit, we make explicit something which was implicitly
assumed before: size must be a scalar. the mask creation code assumed
size was a scalar and would break if it wasn't, but nowhere was that
expectation made explicit; it now is, in documentation for
log_polar_grating and with an assert in that function and
create_sf_maps_cpp (which create_masks relies on)

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