add method to load a `WorkUnit` after being lazily initialized

[maxwest-uw]

might help with workflow stuff and is generally good for testing.

use pattern

wu = kb.work_unit.WorkUnit.from_sharded_fits("filename", "directory", lazy=True)
...
# do some other stuff, like read manipulate metadata
...
# now I want to load in the `ImageStack`
wu.load_images()