Enum member isSummary
Tests whether a struct/class has the necessary information for calculating a T-test. It must have a property .mean (mean), .stdev (stdandard deviation), .var (variance), and .N (sample size).
enum isSummary(T)
= is(typeof(T .init .mean)) && is(typeof(T .init .stdev)) && is(typeof(T .init .var)) && is(typeof(T .init .N));