Cov2D src should use image accessor

[garrettwrong]

ASPIRE-Python/src/aspire/denoising/denoiser_cov2d.py

Line 170 in 783e95d

def images(self, istart=0, batch_size=512):

Our other sources have been converted to .images[] via the ImageAccessor layer. Seems like we just missed this one.

[garrettwrong]

This has been converted in #1014 . Closing.