fix: return the value as all three quartiles for single-element input

Quartile called Median on empty half-slices when the input held one value
and discarded both errors, so it returned {NaN, v, NaN} with a nil error.
The NaN spread silently through InterQuartileRange, Midhinge and Trimean.

A single observation is its own lower hinge, median and upper hinge, which
is what Percentile already returns for the same data. Return that instead,
and note why the remaining discarded Median errors cannot fire.

DOCUMENTATION.md

@@ -2183,6 +2183,9 @@ func Quartile(input Float64Data) (Quartiles, error)
 ```
 Quartile returns the three quartile points from a slice of data
 
+A single value is its own lower hinge, median and upper hinge, so all
+three quartiles are that value.
+