added some keywords of model pde


git-svn-id: svn://svn.code.sf.net/p/elmerfem/code/trunk@6786 04224531-1f84-46d8-96ce-875b9e37dce3
added nonlinear iteration (fix point iteration) to model pde.


git-svn-id: svn://svn.code.sf.net/p/elmerfem/code/trunk@6787 04224531-1f84-46d8-96ce-875b9e37dce3