Restructure IO routines

[Rohit-Kakodkar]

Restructure input routines

Tracking this issue through branch #issue-108

• Move the constructor for mesh struct mesh(const std::string filename, const specfem::MPI::MPI *mpi); to a function IO namespace since this constructors reads an external database file. The IO function should return specfem::mesh::mesh instance. #146
• Move function specfem::sources::read_sources(args...) #150
• Move function specfem::receivers::read_receivers(args...) to IO namespace #169

[Rohit-Kakodkar]

@lsawade suggested by lucas

[lsawade]

Working on this on branch #issue-108

• Move the constructor for mesh struct mesh(const std::string filename, const specfem::MPI::MPI *mpi); to a function IO namespace since this constructors reads an external database file. The IO function should return specfem::mesh::mesh instance.
• Move constructor for sources to read_sources
• Move constructor for receivers to read_receivers