calculate maximum gaps in measurement for a given y-interval

[myoung3]

Internal kaufman-lab processes use this as a criteria to determine if an average meets "completeness" requirements.

maximum gap is defined as the maximum length in a y interval of any concurrent run of time-points for which a specific x value is missing. Missingness could be either structural (ie part of a y interval isn't in x at all) or explicit (a row in x/specific x interval has NA for a value)..