Allow attributes to be set on niftitodicom command line

[dgobbi]

Many of the dicom tools (dicomtocsv, dicomfind, etc) allow attributes to be set via "-k" options:

-k StudyDate=20120706

For those other tools, this is used to specify a query. For niftitodicom, could it be used to specify attributes for the output data set? Questions:

• Will these override attributes from the "source" dicom files? Obviously, yes.
• Will these override attributes set by the generator, such as PixelSpacing or SOPInstanceUID? Maybe best not to.
• What about setting values that will vary, e.g. be different for each slice? Defer.

This idea could be taken further by creating a grammar for writing "rules" for filtering the metadata, e.g. like the RSNA-CTP deidentifier. Then we could filter the source metadata in order to produce output metadata, instead of simply replacing values.

For now, though, simple replacement is enough.

Edit: Instead of using "-k", it might be best to use "-a" to specify attributes that are to be added, and perhaps "-e" to specify attributes that are to be deleted.