Add a new getIncomingSet() to AtomTable

This will be used to provide incoming sets, so that they
are not stored with the atom any longer.
Get incoming set from the atom table not the atom.

In the future, incoming sets will only be stored by the
atom table, not the atoms themselves.  This will allow atoms
to become immutable.
Add a new HandleSeq index.

This index is in the opposite direction from the existing
HandleSAeqIndex.  This will be used to implement incoming sets.
Add implementation of an incoming index.

Will be using this in the atom table, shortly.
atomspace: Several fixes to recently added code.

Unit tests are failing; make fixes to get them to pass.
atomspace: partial elminiation of incoming set.

This breaks one of the pattern matcher UTests. Will investigate
this later, after the conversion is complete.
atomspace:  remove incoming links from atoms

This completely removes the incoming link structure from atoms,
and simplifies a lot of code.  Unfortnately one unit test crashes,
investigating next.
work around bug in importance index.

It appears that someone somewhere is removing atoms from the TLB,
but failing to remove them from the atom table.  This is causing
atoms to appear in the incoming set, even though they supposedly
don't exist any more. Ooops.
pattern matcher: fix unit test.

Changes to the atompsace implementation caused this unit test to fail.
But that's because the unit test is poorly designed, and was testing
the order of something where the order doesn't matter.  i.e. the
test works today, may break in the future, again.
Merge pull request #127 from linas/outgoing

Redesign how incoming sets are handled in the atomspace.

All unit tests pass. This design change is needed for planned performance enhancments and some general code simplification.