ABI issue with CONDUIT_INDEX_32

[cyrush]

We advertise CONDUIT_INDEX_32 as a compile time option, and that implies that downstream codes can make a selection during tier build time.

That was the case in the early days of conduit, however this longer works.

If a downstream code defines CONDUIT_INDEX_32 when using conduit headers, the ABI will not be consistent with a build of conduit where it was not defined.

To properly support CONDUIT_INDEX_32, it should be a cmake configure option that creates a define in conduit_config.h that is used consistently during build time and by downstream codes.

All of that said, we always recommend index_t as a 64-bit signed integer (this is the default configuration )