Automatic Z-coordinates.

[morganjwilliams]

For 2D coordinate systems (e.g. from an image), autopew currently calculates the transform for x and y coordinates, but imposes a default value for z.

This value for Z could in the first instance be extracted from the mean of the reference points, or in the case of direct coordinate transforms (see #2) could be an interpolated surface over x-y. These values for z should be bounded by stage limits (see #1).