cmake: added HL library to build
Frame: added API to report number of bytes in image or chunk
Writer: first stab at direct chunk writing

Works only under certain conditions: mainly the test dataset *must* be
configured as a single chunk!
Tweaking closing of hdf objects

[skip ci]
Writer specify writing unsigned int32
Adding testdata dir
Added ability to switch on/off the direct chunk write
Merge pull request #3 from ulrikpedersen/direct_chunk_write

Direct chunk write