Draft: Asynchronous parallel I/O for reading and writing wavefield data.

• Added a class which handles the I/O using:
  1. Asynchronous POSIX-IO
  2. Nonblocking MPI-IO
  3. ADIOS2-IO
  4. Current POSIX-IO implementation
• Added WAVEFIELD_DIR variable, which points to the directory where the wavefield data is going to be stored.
• Removed the code which removes the wavefield directory without any proper checks.
• Added proper asynchronous communication routines.
• Added buffered I/O for functions like outmod,outgrad and outhess. Added mpi versions for the same functions.
• Added buffered I/O for functions like readmod, and mergemod.
• Improved the performance of gradient_T and gradient_T_acoustic by adding explicit simd pragmas.
• Improved the communication in the Hessian stacking using a single MPI_Allreduce instead of an explicit call for each array field.
• Improved error handling in the case of POSIX IO, replaced the call to perror with a call to log_fatal.