Added the new subroutine open_ASCII_file to open text files that can be read
or written to using simple Fortran read or write statements.  This was
previously handled via a common open_file call, but FMS2 is changing the
interface for this routine, and creating this new file de-conflicts most the
MOM6 calls to open_file from these altered interfaces.  The existing routine
open_file is unaltered and works exactly as before.  A number of existing calls
to open_file were also modified to use this new interface, in some cases where
only the root_PE was writing, this includes altering the code so that only the
root_PE actually opens the file.  All answers and output are bitwise identical.

  Added the overloaded interface rescale_comp_data to MOM_domains_infra to
rescale the values in the computational domain of a 2-d, 3-d, or 4-d array by
some factor without having to know the index convention that those arrays used.
Also simplified some of the code in MOM_io_infra by making use of
rescale_comp_data, and added a new optional scale argument to the various
MOM_write_field routines.  All answers are bitwise identical, but there are new
interfaces and optional arguments for existing routines.

  Use the new scale argument to MOM_write_field to simplify write_vertgrid_file
and write_ocean_geometry_file.  All output and answers are bitwise identical.

  Added and used the file_type as the handle for input and output, changing from
using an integer.  This changes the type of one of the required arguments to
write_field, write_MOM_field, create_file, reopen_file, get_file_info,
get_file_times, get_file_fields, write_metadata_axis and write_metadata_field,
and added new variants of open_file, close_file and flush_file using this new
type.  Also added the new routine file_is_open.  All answers are bitwise
identical, but there are changes to multiple I/O interfaces.

  Added the new optional scale argument to the real versions of the
read_variable routines.  All answers are bitwise identical, but there are
new optional arguments to public routines.

  Revised the depth list type used in MOM_sum_output to accommodate I/O using
ordinary routines, and to make the code creating, reading and writing it
potentially separable from the MOM_sum_output module by avoiding the use of the
MOM_sum_output control structure in most of the routines.  Also changed the
index 'l' to 'li' in some places to avoid having it misread as '1'.  All answers
are bitwise identical but there are changes in some subroutine arguments.