Auto detect and warn against performance pitfalls

[avik-pal]

Implementation

Set up a preferences-based system for doing the following:

1. default: Do nothing, i.e., the current state
2. warn: If we detect type mismatches, we display a warning and tell users to use Experimental.@debug_mode to figure out the exact location of the model. (I don't mind having this as the default but this will be breaking if we choose to do so)
3. autoconvert: My least favorite version (Flux's default), but few people do seem to like this. We convert the types to the most performant version.
4. error: Throw an error if eltype mismatch is detected.

Documentation

1. Document the preferences for eltype detection
2. Create a Page showcasing the various options, even from GPUArrays verse
3. Options from DispatchDoctor once that is merged into LuxCore