Move DataType into its own .hpp/.cpp file combination
Charon: Implement TypeSpec for DataType
PSize: change guards from HSIZE to PSIZE
DataSet: First skeleton-like implementation
Add PSize to pandora.hpp
DataSet: Add extend function
Add skeleton DataSet test (TestDataSet)
PSize: add fill(hsize_t) function
DataType::data_type_to_size: return in bytes not in bits
DataSet::guessChunking: add function (port from h5py)
TestDataSet: add tests for chunking