"stop_before_pixels" argument for "get_identifiers"

[allanhummer]

Sometimes image information of the DICOM is not needed when processing the headers (e.g. for grouping multiple measurements of the same patient). Especially when dealing with large quantities of DICOMs, it would be very useful to omit the pixel array to save a lot of memory.

read_file (dcmread), which is used by deid already has a "stop_before_pixels" argument. It would be great to access this directly when using "get_identifiers".

Thanks for providing this helpful toolbox!

[vsoch]

Sure, that sounds easy to do! Here is a PR #164 for you to test, and I've added some questions I'm interested in your feedback on.