Version 2 - **API breaking**

[taobrienlbl]

• [x] updates/upgrades to the API; dependent codes will need to be updated with new kwargs
• [x] make code pass linting
• [x] add option for xarray output of fastkde.pdf() (will become default if xarray is present)
• [x] add linting checks to github actions
• [x] add code tests to github actions