allow RealSet construction from RealInterval

[rwst]

The current restriction makes reuse of RealIntervals difficult and makes no sense anyway.

sage: ri=RealInterval(10,11)
sage: RealSet(ri)
---------------------------------------------------------------------------
/home/ralf/sage/local/lib/python2.7/site-packages/sage/sets/real_set.pyc in __classcall__(cls, *args)
    634                 intervals.extend(arg._intervals)
    635             else:
--> 636                 raise ValueError(str(arg) + ' does not determine real interval')
    637         intervals = RealSet.normalize(intervals)
    638         return UniqueRepresentation.__classcall__(cls, intervals)

ValueError: 11.? does not determine real interval

Not to be confused with #17799 which is about sage.sets.real_set.RealInterval.

CC: @mkoeppe

Component: basic arithmetic

Issue created by migration from https://trac.sagemath.org/ticket/17791

[rwst]

Description changed:

--- 
+++ 
@@ -13,4 +13,4 @@
 
 ValueError: 11.? does not determine real interval
 ```
-
+Not to be confused with #17799 which is about `sage.sets.real_set.RealInterval`.