Issues on Interval class

[Diego Manhães Pinheiro]

The Interval class found on the oship.openehr.rm.support module must be updated with these enhancements:

 - Separate the tests from the doctstring class, including as a unittest, covering all possible operations.
 - Substitute the Smallest and Largest class to the Inf and -Inf IEEE 754 float point constants.
 - Minimize the number of type checking, verfiying just the if object has the same type and if the object is a infinite.
 - Remove the possibility of define a Interval with both bounds as infinite.
 - Remove the possibility of define a Interval just with a bound. Always both bounds must be passed as arguments.
 - The default values of lower_included and upper_included must be True.
 - Implement the in operator with the same behavior from has operation.
 - Use the Persistent class instead of grok.Model in order to maintain the class as simple as possible.