Allow configuration of default properties of blosc compressor

[joshmoore]

Originally opened as bcdev#42

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The goal of this PR is to allow the user to set the number of threads used for blosc decompression on the auto-created Compressor objects during read operations.

This is done via CompressorFactory.BloscCompressor.defaultProperties.put("nthreads", 8);

This was the least invasive/modifying approach I could find (in conjunction with PR bcdev#41 for user defined compressor objects). However, it does expose the actual BloscCompressor class and the implementation of the map. I'd be happy to implement something more extensive keeps the class implementation private if the maintainers prefer.

I also only updated the nthreads property to pull from the defaultProperties map rather than the default constants. If this approach is acceptable, I can change the other properties to have the same behavior.

[joshmoore]

Thanks again for this, @pedson. Merging for 0.4.0 but two things occur to me:

• there's no example usage/docs/tests
• it feels slightly like a backdoor

Those two taken together might balance each other out. 😄 Before this becomes too ingrained, it could likely use some more discussion (e.g. add interface methods for modifying, etc.)