MS-FO-LC1-017: cast untypedAtomic to other type with arithmetic and compare ops.

Class: Editorial      
 
Why don't we cast untypedAtomic to other type instead of directly to
xs:double and use xs:double only as fallback for arithmetic and compare?

 
This would mean that an expression such as xs:untypedAtomic(1)+1 results
in an integer(2) instead of a double(2.0e0) as it does now.
 
It would also be more consistent with the handling of untypedAtomic when
passing it to a function.