fix 3 bugs in navierstokes.cpp
fix 3 bugs in navierstokes.cpp

- fix empty string passed into TimeIntegration factory
- fix wrong top limit in a for-loop of diffusion terms
- fix unspecified object names for restarting data
- improvement: force absolute path in HDF5 file

mesnardo/PetIBM,barbagroup/PetIBM,piyueh/PetIBM,mesnardo/PetIBM,mesnardo/PetIBM