fixed typos [ci skip]
Reader class implements RecursiveIterator
added ChainReader class
code clean-up
refactored Reader class to remove Callback usage
refactored Writer class to remove Callback dependence
refactored StreamFilterManager to remove Callback dependence
simplified StreamFilterManager tests
added package config file
ScalarField::$fixed attribute is now public
refactored ArrayField::decode() method
added AggregateCallback class
fixes for AggregateCallback
added AggregateReader class
tweaked AggregateReader
renamed AggregateCallback class to CallbackReduction
added AggregateWriter class
updated comments and tutorial.md
refactored CallbackReduction
tweaked AggregateReader and AggregateWriter