ISs that define a coloring of something, such as a graph defined by a sparse matrix
typedef struct _n_ISColoring *ISColoring;
One should not access the *is records below directly because they may not yet
have been created. One should use
ISColoringGetIS() to make sure they are
created when needed.
When the coloring type is
IS_COLORING_LOCAL the coloring is in the local ordering of the unknowns.
That is the matching the local (ghosted) vector; a local to global mapping must be applied to map
them to the global ordering.
This is not a