clarify and implement the precedences of I/O hints

[wkliao]

Clarify and implement a new I/O hint precedences

* 1st priority: hints set in the environment variable `PNETCDF_HINTS`, e.g.
  `PNETCDF_HINTS="nc_var_align_size=1024"`. Making this the first priority
  is because it allows the same application executable without source code
  modification to run using different settings through a run-time
  environment varaible.
* 2nd priority: hints used in the arguments of `ncmpi__enddef()`, e.g.
  `ncmpi__enddef(..., v_align=1024,...)`. With the same reason as described
  above, application source codes making a call to APIs using specific
  arguments should have a priority lower than the ones set in a run-time
  environment variable, so that users can try different hints using the
  same executable.
* 3rd priority: hints set in the MPI info object passed into calls of
  `ncmpi_create()` and `ncmpi_open()`, e.g.
  `MPI_Info_set("nc_var_align_size", "1024");`. This is because MPI info
  can only be passed at the file creation or open time whose hints are
  generally applied to the whole operation to the fille until it is closed.
  However, `ncmpi__endde()` can be called multiple times between a file's
  open and close, as users may want to use a settings unique for individual
  call to `ncmpi__endde()`. Thus `ncmpi__endde()` should have a higher
  priority than MPI info.