Updating setup and adding tests

[MJWeberg]

Clean-up and behind-the-scenes updates. No changes to the functionality of the code was made.

Summary of changes:

• Finally updated the package to use a pyproject.toml for setup rather than the deprecated setup.py
• Removed the unused conda.recipe feedstock (closes Move conda recipe to separate feedstock repository #17)
• Added parameter checks and user warnings when fitting templates are loaded with incompatible wavelength ranges and centroid values.
• Added more unit tests