More lags in the external VFCI

[matdehaven]

Add option to hav more lags in the external vfci (either lots of options or a function to estimate the external vfci on demand).

[matdehaven]

Finished rewriting the external VFCI estimation into a function that can be called with various options.

Report comparing VFCI estimations

Takeaways:

• Changing the y variable doesn't affect much -> 'output' as our default
• Shifting the forward horizon does change a lot
• Adding lags changes little, even for large horizons
• Including or excluding the lags from the VFCI calculation matters little
• The number of principal components included has an effect, but they are all substantially similar