Data Structures
Here are the data structures with brief descriptions:
buffer  Simple buffer structure 
dmatrix  Structure for both dense and sparse matrix.
 if the matrix is dense, then nz is set to 1 and idx, jdx are set to NULL.
 if the matrix is sparse, then nz is set to the number of nonzero elements and the coordiante is saved at idx and jdx

pcg_status_t  Contains pcg status information 