Allow specification of Python virtual environment

[Debilski]

Hi,
we are running into some issues with conda libraries when using scVIIntegration (Version `OPENSSL_3.2.0' not found, presumably because conda libraries expect a newer version than what we have here.). When changing the line

reticulate::use_condaenv(conda_env, required = TRUE)

to

reticulate::use_virtualenv(python_env, required = TRUE)

(with a properly installed Python virtualenv) it seems to work for us.

Is there any deeper reason that only conda is possible by default or would it make sense to add an additional argument python_env to scVIIntegration?