Merge branch 'devel'
clean code and fix fluid-solid forward simulation

--move the fluid-solid coupling into compute_coupling_acoustic_el.f90
--change Makefile.in to include compute_coupling_acoustic_el.f90
--implement weak fomulation of PML for fluid-solid forward simulation
Merge branch 'devel' to includ new modification
Merge pull request #12 from xiezhinan/devel

clean code and fix fluid-solid forward simulation
Merge branch 'devel'