[Dcmlib] NEW FEATURE: TagKey is now a union of uint16_t or uint32_t

[Mathieu Malaterre]

Hello,

	since the only problem with my patch was solved recently by JP (no more 
notion of 'Extended' keys), I was able to commit the patch. I did check 
that python wrapping is still compiling. Please do not hesitate to let 
me know if this break anything.

	Also let me know how faster your code now works, I could get a ~25% 
speed increase when doing lot of access to the dictionary.

Thanks
Mathieu