no progressCallback in main functions like retrieveStudy

[Christian-e-S]

Currently _httpGetMultipartApplicationDicom is called within the major functions like retrieveStudy, with progressCallback hardcoded to "false".

Example: return this._httpGetMultipartApplicationDicom(url, mediaTypes, false, false, withCredentials);

It would bring a lot of value if one could handover a progressCallback via the "options" parameter to retrieveStudy. Without that, we cannot show any progress of the download to the user.