add-cluster: resize moles to 512, refactor
add-cluster: resize moles to 512, refactor

Extract variables for 'montage_size' and 'mole_size', set 'mole_size' to
512 for more managable files. Finer detail on the images are not
required - the microscope images will give us this.

Test Plan:
Try to add a new cluster with the 'mel add-cluster', new size images
written as expected.

aevri/mel,aevri/mel