Domain decomposition without the agonizing pain

This note is intended as a first introduction to domain decomposition programming with the PETSc library, specifically the DMDA component. This note goes with dm/ex13f90 in PETSc.本笔记旨在作为使用PETSc库开展区域分解编程的入门介绍，重点聚焦于其中的DMDA组件。本笔记与PETSc中的dm/ex13f90示例配套使用。