TODO

* Flip y-axis in show.  Flip axes.

* Go through how RNG:s are seeded, especially when one CS is used
  repeatedly as component of other CSs

* Go through use of and rename classes Finite and FiniteMask.

* Avoid intersection at top of MaskPartition

* Implement CSetPartition

* Implement lazy evaluation in BinaryCSet.makeValueSetMap

* Implement SubMaskContainer (startIteration etc) to avoid code duplication

* eliminate code duplication in _elementary.FanInRandomMask

* loosen up criteria on Finite connection sets to BoundedConnectivity
  (Masks with upper bounds on the number of connections per source or
  target.)

* generalize FanInRandomMask to work on BoundedInConnectivityMask:s, for
  example block masks of BoundedInConnectivityMask:s

* generalize SampleNRandomMask to work on Finite masks

* make value sets returned by value(c,k) iterable