define IGWN environment that is independent of python version

Currently all pre-defined IGWN environments are python version specific, e.g. "igwn-py38", or "igwn-py39" being pinned to python 3.8 and python 3.9 respectively. This means that for normal users who want to just track a single environment for basic LIGO software usage there are two unfortunate issues:

• users have to pick a python version, which for most users will be an unfortunate decision they won't know how to make
• the environment will necessarily expire at some point in the future when the python version they chose expires and the environment is no longer updated.

A new generic environment should be defined that is not python version specific. This would allow basic users a clear choice of environment that they can track in perpetuity.