fixes compilation of xmodel_update when configured with ADIOS
removes obsolete links in README file
Reverted commit of wrong version of submodule m4

Commit a305b99e mistakenly reverted the submodule m4 to an earlier version. This commmit reverts that mistake.
adds error checking when reading of Par_file parameters; updates user output in model_tomography.f90
renames output from xmeshfem3D to output_meshfem3D.txt (to avoid overwriting by xgenerate_databases); updates error messages
Merge branch 'devel' of https://github.com/geodynamics/specfem3d into devel
added calculation of the estimated day and time at which the run will finish; taken from SPECFEM3D_GLOBE; useful for large runs