midpoint(x, ...)# S3 method for default
midpoint(x, ...)
# S3 method for numeric
midpoint(x, ...)
# S3 method for waveband
midpoint(x, ...)
# S3 method for generic_spct
midpoint(x, ...)
# S3 method for generic_mspct
midpoint(x, ..., idx = !is.null(names(x)))