Test BitGroom/BitRound h5repack with float data

[ajelenak]

Both plugins' ` callbacks remove themselves from the pipeline on
non-float datasets, so the previous shared h5repack_layout.h5 input (integer
data) caused the tests to silently apply nothing once the plugin path was
fixed in #238.

Each plugin now ships a small h5repack_floats helper that
generates a chunked dataset of non-integer floats, and two h5dump checks
on the post-repack output assert that the filter ID stays in the pipeline
(via the -pH command options) and that the dumped values match the captured
quantized output (via a .ddl output).

Closes #240.

[brtnfld]

add_executable placed outside if (H5PL_BUILD_TESTING) — minor

In both BITGROOM/example/CMakeLists.txt and BITROUND/example/CMakeLists.txt, the h5repack_floats_* executables are compiled unconditionally, even when H5PL_BUILD_TESTING=OFF. Unlike the example programs that live before the if (H5PL_BUILD_TESTING) block (which are independently useful), these generators serve no purpose outside of testing. Consider guarding them:

if (H5PL_BUILD_TESTING)
  add_executable (h5repack_floats_bitgroom ${PROJECT_SOURCE_DIR}/h5repack_floats.c)
  ...
endif ()