schord is a function used for post-processing the result of
schur. The list argument is understood to be the return value
from schur, and must contain elements t and z.
The result of schord is a list containing elements t and
z. It is possible to concatenate use of schord.
The basic idea of schord is to rearrange the t matrix to
move the diagonal block in the row given by the first scalar argument
to the row given by the second scalar argument.