Revamp the interface for timestamps

[jeremykubica]

Fixes #350

Previously KBMOD stored the timestamps in multiple locations, providing the potential for out of sync data or other errors (especially as we allow changes at the RawImage level). This PR consolidates all of the timestamp storage in a single location. Specific aspects include:

• Remove the (zero-based) timestamp array from ImageStack. This could easily get out of date.
• Provide methods for accessing obstime or zeroed-time from ImageStack. Make them clearly named in terms of what they return.
• Provide a function to build an array of zeroed timestamps from ImageStack (at linear cost). This is used in a few locations in the search stack and tests.
• Also removes the ability to overwrite images in the ImageStack (only needed for testing before we changed the return policy).

Note that this PR involves a tradeoff of extra computing time in a few cases to avoid carrying around multiple copies of the timestamps.