Update IBI allocation relative to ITC

[brtietz]

From Michele's review of our IBI/ITC implementation:

Consider subtracting 100% of the IBI from the ITC basis rather than adjusting IBI by the manual allocations to different depn terms.

This suggestion matches the example in [https://www.energy.gov/eere/solar/federal-solar-tax-credits-businesses] and is something we should look into for the release.