ServiceObject.IsValid() method

[Ivan R. Judson]

> What exactly do you mean by validating? Is it that a URL points to a 
> service of the "type" you are expecting?

Mostly that there is something at the other end of the url. Even if it's not
the type object you expect, it's like validating a url -- you can tell if
it's correct or not without getting the data.
 
> we could define some validation of that sort, it requires 
> agreeing on how 
> to describe type names, and that you will pay the round-trip 
> price for the 
> validation.

Tom implemented a simple ping method that returned one. That's the kind of
thing I'm thinking about, but at the ServiceObject level, so that any
objects that are service objects can be checked remotely.

Sure there's a performance hit, but I'd rather take up the performance
problem with the Globus/pyGlobus folks and have our code work "more
correctly" than work around the issue. 

I think it's trivial to implement enough to be sufficient.

--Ivan