Remove some levels of indentation.
Add "binary" format case. (not implemented.)
Exit if DataArray "format" is not known.
Add checks of other zlib error return codes.
Remove getXmlAttribute() and findDataArray from class interface.

This functions have nothing in common with BoostVtuInterface.
Move up readDataArray() for reading CellData/MaterialIDs.

Other DataArray will follow.
Use readDataArray() for reading Points.
Use readDataArray() for reading CellTypes.
Use readDataArray() for connectivity.

readElement() is not required in the interface and takes an iterator instead of input stream now.
Strictly check DataArray types.

This requires modification when reading ascii because of
Add decoding of base64 binary data in readDataArray().
Add getRequiredXmlAttribute().
Remove unnecessary indentation by rearranging ifs.
Move up readAsciiData(). Generalize to std::istream.
Move up decodeBinaryData(). Generalize to std::istream.
Check input stream state while decoding from base64.
Make readDataArray() return optional<> in case of error.

This allows an early exit in case of incomplete data.