missing doxygen in libdap4

[edhartnett]

Improve Doxygen documentation in libdap4

Summary

The libdap4 subdirectory implements the DAP4 (Data Access Protocol 4) client
for netCDF-C. Prior to this work the directory had almost no Doxygen
documentation: no @file blocks, no struct/enum field comments, no
@param/@return tags on public functions, and no macro documentation.
This issue tracks a systematic pass to bring the directory up to the same
documentation standard as the rest of the library.

Changes made

Header files (12 files)

File	Changes
ncd4types.h	@file block; all structs (NCD4node, NCD4meta, NCD4parser, NCD4response, NCD4curl, NCD4INFO), enums (NCD4sort, NCD4translation, NCD4mode, NCD4format), union ATOMICS, and all macros documented with /**< */ field comments and /** */ block comments
ncd4.h	@file block; all EXTERNL function declarations documented with @param/@return; all macros (NCCHECK, FAIL, BUILDOFFSET, INCR, DECR, OFFSETSIZE, TRANSFER, DELTA, MARK, GETCOUNTER, SKIPCOUNTER, PUSH, nullfree, nulldup, d4alloc, id/accessor macros) documented
ncd4dispatch.h	@file block; all dispatch entry-point declarations (NCD4_open, NCD4_close, NCD4_abort, NCD4_inq_dim, NCD4_get_vara, NCD4_get_vars, NCD4_initialize) documented with @param/@return
d4util.h	@file block; d4size_t, D4blob, NULLBLOB, OFFSET2BLOB, BLOB2OFFSET, byte-swap macros (swapinline16/32/64), data-content flags (HASNIL, HASSEQ, HASSTR, HASOPFIX, HASOPVAR, LEAFSEQ), and ncd4__testurl documented
d4odom.h	@file block; D4odometer struct fields and all odometer functions documented
d4debug.h	@file block; debug-flag #undef macros, PANIC/ASSERT, MEMCHECK, THROW/THROWCHK, d4breakpoint, d4throw, LOG0/1/2, NCD4_sortname, NCD4_subsortname, NCD4_debugcopy documented
d4bytes.h	@file block; D4bytes struct fields and all functions/macros documented
d4curlfunctions.h	@file block; CURLFLAGTYPE enum, CURLFLAG struct, and all curl-option functions documented
d4chunk.h	@file block (placeholder header)
d4http.h	@file block; all legacy HTTP function declarations marked @internal
d4read.h	@file block; legacy read declarations marked @internal
d4includes.h	@file block describing its role as the master include file

Source files (21 files)

All 21 .c files received a @file block with a brief description of the
functions each file implements:

d4bytes.c, d4chunk.c, d4curlflags.c, d4curlfunctions.c, d4cvt.c,
d4data.c, d4debug.c, d4dump.c, d4file.c, d4fix.c, d4http.c,
d4mem.c, d4meta.c, d4odom.c, d4parser.c, d4printer.c, d4read.c,
d4swap.c, d4util.c, d4varx.c, ncd4dispatch.c

The following key public functions also received full @param/@return
Doxygen blocks directly above their definitions:

• NCD4_open (d4file.c) — DAP4 URL open entry point
• NCD4_dechunk (d4chunk.c) — chunked wire-format decoder
• NCD4_parse (d4parser.c) — DMR XML parser
• NCD4_metabuild (d4meta.c) — NetCDF-4 substrate metadata builder
• NCD4_processdata (d4data.c) — endian swap + checksum verification
• NCD4_convert (d4cvt.c) — type-conversion utility
• NCD4_fetchurl (d4http.c) — HTTP fetch via libcurl

Style notes

• Single-line /** text. */ comments are used without a redundant @brief
  tag (Doxygen treats the first sentence as the brief description automatically).
• Multi-line block comments use @brief only when an explicit brief/detail
  separation is needed.
• Internal-only helpers are tagged with @internal.
• Macro groups are wrapped in @defgroup / @{ / @} where it adds value
  (e.g. chunk-header flags, special attribute names, data-content flags).