Add chkAlg field to OrigDatablock

[jl-wynen]

Datablock has a chkAlg field to encode the checksum algorithm used for its data files but OrigDatablock lacks this field. As it stands right now, the the data files in an original datablock can have a checksum but some convention or external mechanism is needed to encode the algorithm.
Please add OrigDatablock.chkAlg.

[nitrosx]

@jl-wynen and myself brainstormed again on this issue.
I'm going to promote this to a must-have feature.