Make image/weight keys for blocks in MosaicFactory user settable

This makes it possible to quickly mosaic alternative versions of blocks
(ie, low res versions).
MosaicResampler saves original pixel scale to DB

See the `native_pix_scale` field.
LockedBlockFactory verifies input paths exist
User accessible n_iter_max setting in LockedBlockFactory
Fix MosaicResampler for case of only one input image
ScalarMosaicFactory lets user set output MosaicDB keys

Thus you can output multiple, parallel versions of a mosaic that don't
clobber each other in the MosaicDB record.