Trac #12892: Toric fibration morphisms
Trac #12892: Toric fibration fiber

Note that the fiber of a toric morphism is a reducible union
of (irreducible) toric varieties. This patch implements the embedding
of fiber components as a scheme morphism.
Trac #12892: Pull back toric divisors via toric morphisms
Trac #12892: Reviewer changes to toric morphisms.
Trac #12892: Move fibration methods to subclass of dominant morphisms
resolved conflict with Trac #14891
Make fiber_generic return component count, doctests not adjusted yet.
Doctests pass, still looking at code.
Throw ValueError instead of TypeError for complicated morphisms.
Use span instead of submodule to deal with rational preimages
Fixes to deal with maps between sublattices and virtual rays.
Treat generic fiber correctly as a fiber component.
Added a long fibration example. One doctest failure due to fan isomorphism bug.
Orthogonal arrays
Balanced Incomplete Block Designs and construction for k=4
Move steiner_striple_system from block_design.py to bibd.py
Orthogonal arrays -- bugfix
Orthogonal arrays -- doctest
initial modification
all test pass