Subtomogram average support now included

This release incorporates the final entity that was incomplete: representing subtomogram averages. A subtomogram average consists of two data types: an indexed lattice, which is a base64-encoded 3D volume with the subtomogram average's density and a list of 'shapes' each of which is a reference to the lattice together with a transform ID specifying how the particle instance is mapped back to the tomogram.