Access the memory contents of a file created by NC_DISKLESS

[DennisHeimbigner]

re: e-support MQO-415619
Provide a method whereby the memory contents of a file created by NC_DISKLESS
can be extracted the time of nc_close(). We could do this for netcdf-3 pretty easily, but
not for netcdf-4 (currently).
For netcdf-4, the only solution I can see is to create our own version of the HDF5 core
driver to add extra extraction functionality.

[DennisHeimbigner]

Relevant:
https://support.hdfgroup.org/HDF5/doc/RM/RM_H5P.html#Property-SetDriver
https://support.hdfgroup.org/HDF5/doc/TechNotes/VFLfunc.html

[DennisHeimbigner]

Proposal:
I am in the process of adding support for retrieving
the final file image for an in-memory file. This would
extend the existing nc_open_mem functionality.

This involves several changes.

1. nc_open_mem() would now allow the NC_WRITE mode flag
   so a chunk of memory can be passed in and be modified
2. nc_create() would now allow the NC_INMEMORY flag to be set
   to cause the created file to be kept in memory.
3. the ability would be provided to obtain the in-memory contents
   of a file at the point it is closed using a new function
   nc_close_mem(int ncid, size_t* sizep, void** memory)
   Note the following.
4. If nc_open_mem is called with NC_WRITE, then a copy of the incoming
   memory would be made internally (dictated by HDF5 functionality).
   If instead, the file is read-only, no copy would be made.
5. The memory returned by nc_close_mem would be a COPY of the current
   contents if the original was writeable (again HDF5 forced).

I also have a couple of thoughts.

1. Instead of nc_close_mem, I could separate out the memory extraction
   from the close operation. Hence one would call something like
   nc_get_mem() and then nc_close() instead of nc_close only.
   I prefer the nc_close_mem solution, but can anyone provide a
   use case where the separate functions is better?
2. I could always make it so that the incoming and extracted memory
   chunks were copies. This would mean that read-only opening
   a memory chunk would pay the copy cost when it did not have to.
   But the always copy soln might be simpler for the user to understand
   and use.
   Note that this is all preliminary and is subject to change if,
   for example, I cannot get some piece of functionality to work.

[DennisHeimbigner]

Closed by pr #879