Old Dicom formats may not be recognized

[Paolo Brandoli]

The algorithm used to identify old Dicom files may fail.

For instance, the following signature is valid but not recognized: 02 00 10 00 55 49 12 00 31 2e 32 2e 38 34 30 2e 31 30 ....

It represents a valid tag 0002,0010 with VR=UI

The algorithm fails only for old dicom that don't have the 128 bytes preamble.

[Paolo Brandoli]

- Fixes #59

→ 97e2382d5beb