generic MEAN and MEDIAN, new function DISPLACE-ARRAY
Objects other than sequences have means and medians (eg probability
distributions, arrays, sequences/arrays wrapped in another object, etc), so
it makes sense to make these functions generic.
DISPLACE-ARRAY is a small utility function that is used in the implementation
of MEDIAN for arrays, but is also of general utility because it makes the
creation of displaced arrays simpler, dispensing with the need to specify the
element type, and offering sensible defaults when one wants vectors.
Also added tests for all the new functions/methods.