Create to_quantity() method for numerical properties

Hi!

I have encountered myself doing the following many, many times:
```
import quantities as pq
myQuantity = pq.Quantity(myOdml.properties['myProp'].value,
myOdml.properties['myProp'].unit)
```

So I wondered this is probably something people would expect odml to do for them. My suggestion would be a method of the property class that apart from doing the above also checks some stuff like:
* If quantities is installed
* If units are specified
* If the units are compatible with quantities units
* If the value is actually numerical
* ...

And returns appropriate warnings/errors when the requirements are not met.

Let me know your thoughts!
Aitor