Compute uncertainty of offset zeropoint

This uncertainty is standard dev(offsets) / sqrt(number of blocks) and
represents the uncertainty in fixing the net sky offset applied to a
mosaic to zero.
Scale offset metadata for resampled images.

Scales both the `offsets` field and the `offset_zp_sigma` field
according to the pixel scale ratio.
Set pixels to NaN before resampling if flag available

If a flagmap is passed to MosaicResampler, than a temporary image will
be made for resampling where all flagged pixels are set to NaN. This
seems like a better route than trying to resample the flagmap itself.
Resampler add_images_by_path will add pix_scale and offset zp

In manually adding images, I had forgotten to propagate the
pix_scale and offset_zp_sigma document fields. Pixel scale is determined
from the header, while offset_zp_sigmas is an argument to provide this
data.
make offsets and offset_zp_sigma field optional for resampling