Default ParticleReader realType parameter to "float".

This means that all PointsPrimitives loaded with default settings are now compatible with our Renderer backends. This makes the Readers much easier to use in Gaffer, and makes it possible to preview particle caches without type specific code.

Fixes #69.
Merge pull request #73 from johnhaddon/particleReaderRealType

Defaulted ParticleReader realType parameter to "float".