MATAIJ = "aij" - A matrix type to be used for sparse matrices. This matrix type is identical to MATSEQAIJ when constructed with a single process communicator,
and MATMPIAIJ otherwise. As a result, for single process communicators,
MatSeqAIJSetPreallocation is supported, and similarly MatMPIAIJSetPreallocation() is supported
for communicators controlling multiple processes. It is recommended that you call both of
the above preallocation routines for simplicity.