Merge pixl_dcmd into pixl_core

[jeremyestein]

From pixl_dcmd/README.md:

The reason for having this as a separate package instead of having the functionality in pixl_core is because orthanc requires Python 3.9, whereas the rest of PIXL is on 3.10 or higher.

Since we're now on 3.11 for everything, I think pixl_dcmd can be merged into pixl_core, to reduce the number of packages we have and make things a bit easier to work with.

[milanmlft]

Note, from #361, it looks like there is some clash between pixl_dcmd/tests/conftest.py and hasher/tests/conftest.py.
Might be worth resolving this together with the merge.

[jeremyestein]

If you're refactoring this area anyway, you might consider moving some of the not publicly called methods in pixl_dcmd/src/pixl_dcmd/main.py into one of the private modules adjacent to it.