Suggestion: Consolidate and rename incomplete Gamma functions

The incomplete gamma functions are named somewhat inconsistently. I'd like to propose a consolidation, that would ultimately result in something patterned after what's in R. Specifically, rename:

incomplete-gamma      -> lower-incomplete-gamma
incomplete-gamma-tail -> upper-incomplete-gamma

and a wrapper around both that allows for a normalised result:

(defun incomplete-gamma (a z &key (normalised t) (lower-tail t))
  "Incomplete gamma integral"
  (let ((igamma (if lower-tail
		    (NET.COMMON-LISP.OCT:incomplete-gamma a z)
		    (NET.COMMON-LISP.OCT:incomplete-gamma-tail a z))))
    (if normalised
	(/ igamma (gamma a))
	igamma)))

As it is I can't write a normalisation/consolidation wrapper because of the name conflict with incomplete-gamma. If you're agreeable, I can make these changes and send a merge request.