Contradiction in Collection documentation

[Tristan Croll]

Minor thing, but...

In docstring for atomic.Collection (and similar statements in method docstrings):

"By design, a Collection is immutable"

In docstring for collection.hash():

"... it is not supported to use collections as keys of dictionaries or elements of sets since they are mutable."

[Eric Pettersen]

Collections are "immutable" in that their contents cannot be modified by API calls. Nonetheless, they can't be used as keys because they can spontaneously change as the items they contain are deleted by external calls. Reassigning to Tom since Collections are his "baby", so he can elaborate in the documentation if he wants to clarify further.

[Tom Goddard]

The current documentation in molarray.py already says:

"Collections are mostly immutable. The only case in which their contents
can be altered is if C++ objects they hold are deleted in which case those objects
are automatically removed from the collection. Because they are mutable they
cannot be used as keys in dictionary or added to sets."

I added a few more comments on Collection methods that mention mutability.