Equivalent to max(x, na.rm = TRUE) except returns NA (rather than -Inf) if
all elements are NA.
See also
Other summary:
OR.mean(),
OR.min(),
OR.mode(),
OR.modefreq(),
OR.rowleft(),
OR.rowmax(),
OR.rowmax.threshold(),
OR.rowmin(),
OR.rowmin.threshold(),
OR.sum()