Parallelizing ply parser about happly

[nmwsharp]

Hi!

I'd definitely be interested in seeing any efforts to parallelize happly, though before merging such code in to the parser I'd have two concerns:

• There are other parts of the happly code which are probably lower-hanging fruit for performance improvements. In particular, ASCII float parsing and irregular memory accesses could both be dramatically improved. One would probably want to tackle those simultaneously to create an actual "high performance" happly.
• We'd need to "guard" it somehow, to the parallelization doesn't add any software burden to users who don't want it. This could probably be done with #ifdef.

For what it's worth, happly is neither the simplest, nor the fastest PLY parsing library out there; instead, happly is focused on being simple, and handling general non-mesh data. If you're interested in high-performance PLY parsing, you might want to look elsewhere, @mhalber has a great benchmark here: https://github.com/mhalber/ply_io_benchmark (though happly has gotten a bit faster since those timings).

Best,
Nick