Fixed typos in comments.
Moved ACR/NEMA data dictionary to a separate file.

Moved retired ACR/NEMA attributes to a separate dictionary file since they
are officially part of the DICOM standard for many years now (see CP-607).

Please note that this commit changes the prefix of some tag constants from
"DCM_ACR_NEMA_" to "DCM_RETIRED_". Also a few tag constants disappeared
(mainly ACR/NEMA group length attributes, which are retired in the DICOM
standard anyway). However, since the old, manually compiled definitions are
still available in a separate file, the old behavior can be restored by
loading "acrnema.dic".
Fixed issue with wrong check on stream compression.

This wrong check had the result that the three uncompressed transfer syntaxes
were proposed in "decompression mode" even if the pixel data could not be
decompressed, e.g. in case of JPEG compression without a JPEG decoder.
Fixed wrong VR for some retired command fields.

Changed VR from "AT" to "LT" for some retired command fields, i.e. attributes
in group "0000". Now, the definitions are consistent with DICOM part 7.