"[E]mployers are not eligible for any partial credit in determining the amount of the penalty assessed. In other words, an employer that offers coverage to 69% of its full-time employees in 2015 or 94% in 2016 will pay the same penalty that it would pay if it did not offer any coverage at all. This means no employer can afford to inadvertently fail the coverage threshold, so careful attention must be paid to the final regulation's definition of full-time employee and the detailed rules imposed for measuring hours of service[.]"