Add the standard deviation of the noise image to the metacal obs ".meta"

[esheldon]

This is useful downstream, e.g. for detection thresholds

[esheldon]

We might even want to correct the weight map based on this. E.g.

var = noise.var()

# if doing fixnoise
weight[:, :] = 1/(2 * var)

# if not doing fixnoise
weight[:, :] = 1/var

this could be a little noisy for small images

[esheldon]

Although we don't currently have a noise image available if not doing fixnoise....