[PROPOSAL] Define a public interface and stop deploying internal headers

[derobins]

PROBLEM

HDF4 deploys almost every header in the source code, making just about everything public.

This unfortunate decision was made long ago and makes maintenance of the library very difficult. Changing anything risks breaking someone's code, making meaningful refactoring almost impossible.

PROPOSED FIX

• Define the "public API" (TBD - but basically what's in the Reference Manual)
• Split the existing header functionality into public and private
• Stop installing private headers

HEADERS

Internal API - not deployed in 4.3.0, but some functionality will move to the public header file

• dfi.h
• hcompi.h
• hdfi.h

Candidate for not being deployed in 4.3.0 (internal data structures, etc.)

• bitvect.h
• dfufp2i.h
• dynarray.h
• linklist.h
• tbbt.h

Deployed by CMake but not Autotools - will not be deployed in 4.3.0

• glist.h
• hqueue.h
• mcache.h
• vgint.h

xdr-related (xdr will be removed in 4.3.0)

• types.h
• xdr.h

HOW WOULD USERS BE AFFECTED?

PRO
➕ Allows refactoring without annoying users or breaking binary compatibility
➕ Better encapsulation

CON
➖ People have almost certainly built code using private HDF4 things
➖ Initial release of this feature will break binary compatibility

NEUTRAL
✔️ People who used "internal" things could continue to use 4.2.16 (perhaps we could make that a maintenance branch)

[derobins]

This will be addressed in 4.3.0 (#425)