Merge petsc_matrix
Try to fix the compiler error on travis
Try to fix the compiler error on travis, add a template argument for a DenseMatrix instance in the test fucntion
Changes with the comments by Nori, Tom and Lars
Change back the name of _d_nz and o_nz
Change DIAGONAL to diagonal in the comments of PETScMatrixOption
More changes according to the comments by Nori and Tom
Ninor changes in the doc. for two functions.
Revise documentation according to the more comments by Nori and Tom
Rectangular PETSc matrix
Remove a contuctor
Revise the documentation of some functions and changed the member names of PETScMatrixOption
Remove one line in .travis.yml
add the removed make test in .travis.yml
Change a test of set and add
Recovery .travis.yml
another line change in  .travis.yml
more change in one line in  .travis.yml
Change one line in .travis.yml
Activate the test of rectangular matrix vector multiplication