xDistIndex:
===========
A test in finalize was checking sizes of packed_id and to_from
containers. Sizes are normally equal. Since now, and in dedicated
test case it was true as insertToFrom method last call was always done
after the last call to insertIndex method. The test that check size was
then logical as insertToFrom resize to_from with the size of packed_id.
If sizes were different this was a problem.

Now this implicit calling order in between  insertToFrom and insertIndex
is to constraining and complex to understand for user. This commit with
a quick dirty fix release this test. Now a message (in debug mode)
appears and to_from is resized to packed_id size if not of same sizes.
This let user interleave insertToFrom and insertIndex the way he wants
(but still with the restriction that insertToFrom only deals with index
 already present in instance).
 A comment in the header was added about that => cf

For now this size coherence checking is no more done but in future we
may thing about doing things differently so that some checking is added
in finalize method.