The specific analysis API classes are now all singletons.
This change will reduce the amount of memory used, as these classes are initialised multiple times
throughout relax, especially in the test suite. The API objects are not used for local storage so
the multiple instance verses singleton design change will make no difference. The singleton design
pattern code has been added to the base class specific_analyses.api_base.Api_base so that all
classes inherit the __new__() method which implements the singleton.
