Calculate quartiles with data from instance of float64data that already loaded

[pararang]

PR for #60

Adding the Quartiles method on the Float64Data's struct without any input/argument, but use the current instance, like we use Mean(), Max(), etc.

[coveralls]

Coverage remained the same at 100.0% when pulling de616c0 on pararang:calculate-quartiles-from-float64data into 8d66cd3 on montanaflynn:master.