Option to skip Pixel Data

[hkethi002]

There are many cases in which a user may want only the header information, and in order to increase the speed and decrease the memory used by the process, skip reading the Pixel data of the dicom file. A new parameter to func (di *DicomFile) Parse(buff []byte) <-chan DicomMessage which provides a tag at which to stop reading the file return only what Elements have been parsed up to that point much like in the pydicom library or the chafey js library