mdsio global-file sections

[jm-c]

This issue is directly related to @timothyas 's PR #359, but I though it would make it easier to reference in a specific issue.

One thing to clarify regarding global-file sections:

Because of the shape of MITgcm slice arrays (i/j,k,bi,bj), there is no issue to read-in global-files into this tiled array: for a slice along X-dir, all bj tiles will get a copy of the value contained in the file.
However, when writing to a global-file, the "bj" dimension (i.e., nSy) needs to collapse or, more precisely, we need to select which "bj" tile holds the correct value to write to the global-file.

This is all taken care of in PR #359, assuming any section is related to an OB section. And this is the reason for adding the argument "slice" to most of the section/slice I/O routines in pkg/mdsio and pkg/rw (and updating the calls).

We could consider an alternative interface:

1. Keep PR 359 changes in mdsio_read_section.F and mdsio_write_section.F, with additional arguments to these routines to be able to write OB sections to global-files. But also recognize the "empty" slice argument to default back to current tiled files for sections.
2. Keep the argument list of current pkg/rw section routines unchanged, meaning they will still be able to write only tiled files for sections. But for the general case of a section that is not necessary related to OB this is all what can be done without knowing which section it is.
3. Add new interface S/R in pkg/rw to deal with section I/O associated with OB section, similar to [READ/WRITE]REC[X/Y]Z_R[L/S] but with additional argument "slice".
   My suggestion would be to put these new pkg/rw S/R in 2 new files (since read_rec.F and write_rec.F are already long enough): read_obsec.F and write_obsec.F, each containing 4 S/R: READ_OBSEC_[X/Y]XZ_R[L/S] and WRITE_OBSEC_[X/Y]Z_R[L/S].

The advantage of this alternative interface:

1. We keep the original interface (i.e., argument list) of current pkg/rw routines, so there is not need to change all the calls right away (e.g., adding the 2 arguments "kLo,kLi" to pkg/mdsio section S/R in PR Update mdsio slice I/O routines #747 only requires modifying 7 files); could pick the most relevant first (e.g., for OBCS pickup files, which are tested when running restart test do_tst_2+2); and once we know it's working well we could make changes (the best would be in a dedicated PR) to S/R calls in the rest of the code.
2. This does not break user customized code that relies on pkg/rw I/O routines.
3. If, in the future, a new type of section is introduced (e.g., for diagnostics) that is not related to OB, the old pkg/rw I/O routine could still be used but only to write tiled files.
   And writing these new sections to global-file would require providing more informations and changes to mdsio_read_section.F and mdsio_write_section.F.