Local standalone conda installation problems on macOS

[ikrommyd]

Installing combine locally on macOS using conda for educational purposes has been a bit difficult.

First of all, after applying the changes in #897 and compiling, combine would run into this issue: https://root-forum.cern.ch/t/problem-with-root-on-macos-error-module-darwin-uses-additional-module-map/47443.
I bumped up the python version to 3.9 and root version to 6.26.4. That automatically updates the clang version in conda and in order to compile combine I also had to add -Wno-unused-but-set-variable to the CFLAGS.
With that I was able to run test/test_interference.py without errors.
I also tried python 3.10 and 3.11 with root up to 6.26.10.

Bumping up the root version however was causing the test to fail with TypeError: cannot instantiate incomplete class 'CombDataSetFactory'

I'm posting all this because you might want to take a look at it and just fixing the conda installation can go into #897 right away.