Auto Density / Exposure target setting

[marcinz606]

Those were the constants that I have tuned to my preferences but I had an idea to make those user-controlable so you can set your auto density and auto grade target based on how contrasty / dense you want your default conversion to look like:

[marcinz606]

merged to main if anyone wants to try it out

[kjm5000]

Super stoked for this. Not only will it make the default conversion need less work, it will also allow us to auto correct any nuances between our setups such as differences in our light source, lens, camera body, etc.

[marcinz606]

https://github.com/marcinz606/NegPy/releases/tag/0.39.0