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Fix issues #62 and #70

Includes small changes in the last year. No code changes for a couple months. Calling this stable :-)

• not single header anymore - too much of a hassle
• custom allocator not quite ready yet. It is checked in, but sparsepp is still using the old allocator (easy to toggle - line 15 of spp_config.h)

The first official release of sparsepp - production ready on linux/macos.

I do not consider it production ready on Windows, because the realloc implementation on this platform seems to waste an inordinate amount of memory when reallocating (and sparsepp does a lot of reallocations), so the memory usage is much greater on Windows than it should be.

To address this issue, I am currently implementing a custom allocator which will fix this memory usage issue on windows, and should also increase performance on all platforms. This will be version 1.0, as it will truly fulfill the promise of low memory usage on all platforms.