Improve how dicom3tools dependency is dealt with.

[zgypa]

Currently, dicom3tools are used to test/verify the conformance of the produced image. There is a mention in README.md to use defaults.py. Could be better to use a .env schema? Or a local config scheme? Maybe default the location of the tools under modules/. The Makefile could automatically download the tool. I think the URL is pretty standard. For example:

https://www.dclunie.com/dicom3tools/workinprogress/winexe/dicom3tools_winexe_1.00.snapshot.20220618093127.zip