BUG: VTKImageIO did not respect endianness for streamed writing/reading

VTKImageIO did not change endianness from native to big-endian when writing
/ big-endian to native when reading files. This resulted in different results
when using a streaming and not streaming pipeline. Changed behavior to
non-streaming behavior.
A test for comparing block reading/writing to streamed reading/writing is
added.

Included proposals form Matt McCormick:
 - moved convenience methods from ImageIOBase to private methods in
   VTKImageIO
 - using quotes instead of brackets for includes of ITK headers
 - using FillBuffer instead of ImageRegionIterator
 - keep own compare function to be able to compare only the buffered region

Change-Id: I184e78b35bab155db47be3733197b39235d9bcf1
Merge topic 'streamingbug'

aab40f1 BUG: VTKImageIO did not respect endianness for streamed writing/reading