basis improvements for Artinian tower rings

[mahrud]

Summary of changes:

• added helper routine inducedBasisMap
• added helper routine inducedTruncationMap
• added tests for a tricky example
• added basis of a matrix of finite length modules
  • this is a much needed feature, to get the change of basis matrix induced by multiplication by a function in an Artinian ring, for instance.
• added basis for ring map of Artinian rings
  • this time the change of basis matrix induced by a ring map of Artinian rings, which is often very useful.
• added tests and docs for basis Matrix
• changed [basis, Variable] to accept a sequence
• fixed a bug in basis of tower rings
  • this is a somewhat breaking change because the default behavior of basis in a tower ring is changing, but it's needed to fix a bug.

The rest are fixes corresponding to the breaking change above:

• changed a line in documentation of basis
• added tests and docs for basis RingMap
• fixed a basis computation in PushForward
• fixed a basis computation in Schubert2
• fixup a basis computation in DGAlgebras
• fixed a basis computation in Book3264Examples
• fixed a basis computation in GradedLieAlgebras
• fixed a basis computation in PushForward