Refactored the Imager data interface

[0.17.0] - 2023-06-24

Added

• A tutorial using asilib.Imager().
• Documentation in contribute.rst describing the asilib.Imager() interface. Simplified the asilib.Imager().__init__() docstring and point to the thorough interface description in the new Contribute section.

Changed

• Refactored how asilib.Imager() deals with single- and multi-image instances. Now, each ASI wrapper function does not need to load a single image if time is specified, asilib.Imager() does this instead.
• Refactored the themis, rego, trex_nir, lamp_phantom, and psa_emccd wrapper functions with the new asilib.Imager() interface.
• Due to this change, the asilib.Imager.__getitem__() method significantly simplified.