saf utils {passthrough,target} {read,write} support

[ejaronne]

add a submodule to 'apply' that either adds a data object to the .passthrough or to the .target root element

saf attest apply passthrough -i existing.hdf --passthrough-data [file || object] -o output.hdf

saf attest apply target -i --target-data -o ...

it will overwrite existing objects

add a read which will 'return the data currently in':

saf attest read passthrough -i myfile

saf attest read target -i myfile

[aaronlippold]

In talking with @camdenmoors we agreed that the primary method on these should be utils rather than under the attest area.

The function name of either apply or write is still open to debate.

# utilities for working with HDF Passthough data 

saf utils passthrough read -i myfile -o my_passthrough.json
saf utils passthrough apply|write -i existing.hdf --passthrough-data [file || object] -o output.hdf

# utilities for working with HDF Target data

saf utils target read -i myfile -o my_target.json
saf utils target apply|write -i --target-data -o ...