Central CLI

[jammont]

Just a simple PR to tie the CLI components that are already implemented together

• Implemented a central CLI
  • All subcommands are now attached to it
  • Calling scripts directly is now deprecated
• Set minimum python version to 3.9
  • Added 3.12
• Swapped build backend from setuptools to hatchling
• Added a github workflow to test building the package
• Moved mosaic.py and spec_io.py into an io/ module

[pgbrodrick]

Thanks James. Can you outline what the cli interface might look like here for both util and mosaic calls? It looks like we might want to:

• move mosaics up a level (for now)
• replace utils with a self-named spectral-util.py entry point
• place all click entry points into spectral-util, which then makes function calls to:
  -- io
  -- band_math (new .py, where we move some wrapped calls from utils.py into)
  -- mosaic.py

I'm still worried that this is more nested than things need to be, but if we go this route the above seems a reasonable structure. Thoughts? If you're imagining a different flow, please outline!