.env file support?

[raph-luc]

Based on idea by @mkundu1: #1616 (item 2)

Related: #1439 (document env vars)

There are many situations where working with .env files could be beneficial:

• They will first of all document and aggregate all the env vars that we currently support
• Using .env can streamline our GitHub CI workflows so we don't have to specify the same environment variables in each .yml file, for example we could instead set up a single .env.github file for that
• To facilitate changing between different PyFluent/Fluent usage scenarios without having to rewrite code or have separate scripts, can instead have a .env.standalone for standalone Fluent client run, .env.development for local Fluent development version, .env.debug with full logging enabled, .env.container for Docker container run, etc.
• We would also no longer need to manually set environment variables in the operating system when executing PyFluent

Initial example of what a .env.example file could look like that documents all env vars (but does not set any values):

.env.example

# Fluent connection
PYFLUENT_FLUENT_ROOT  # E:\ANSYS Inc\v241\fluent
SERVER_INFO_DIR  # directory where to write server info file
PYFLUENT_SHOW_SERVER_GUI  # 0, 1
PYFLUENT_FLUENT_IP  # 127.0.0.1
PYFLUENT_FLUENT_PORT  # 61738
PYFLUENT_TIMEOUT_FORCE_EXIT  # OFF, 5, 10, 30
ANSYSLMD_LICENSE_FILE  # license file location or license server port@address
PYFLUENT_FLUENT_DEBUG  # 0, 1; whether to launch Fluent in debug mode

# Fluent container launch
PYFLUENT_LAUNCH_CONTAINER  # 0, 1
FLUENT_IMAGE_NAME  # ghcr.io/ansys/pyfluent
FLUENT_IMAGE_TAG  # v23.2.0
FLUENT_CONTAINER_IMAGE  # ghcr.io/ansys/pyfluent:v23.2.0
PYFLUENT_CONTAINER_MOUNT_PATH  # /mnt/pyfluent

# PyFluent debug logging
PYFLUENT_LOGGING  # OFF, DEBUG, INFO, WARNING, ERROR, CRITICAL
PYFLUENT_HIDE_LOG_SECRETS  # 0, 1
PYFLUENT_WATCHDOG_DEBUG  # OFF, ON
PYFLUENT_GRPC_LOG_BYTES_LIMIT  # 1000; 0 to disable

# Documentation specific
DOCUMENTATION_CNAME  # nocname.com
PYFLUENT_SKIP_EXAMPLES_DOC  # 0, 1

# Testing specific
PYFLUENT_TEST_NAME  # none

We can include more Fluent-specific env vars, such as FLUENT_NO_AUTOMATIC_TRANSCRIPT which would help address e.g. #1765

Ideas of how we can actually load the .env files in PyFluent/GitHub:

• Automatically load a local .env (if it exists) whenever PyFluent is imported through python-dotenv (https://github.com/theskumar/python-dotenv), so for example before running GitHub CI we would rename .env.github to .env. PyFluent would then automatically pick up and load as environment variables any local .env file when it is being executed (probably not ideal, maybe if the one automatically loaded was called .env.pyfluent)
• Could instead (or in addition) make PyFluent automatically load a .env file located in %LocalAppData%\Ansys\ansys-fluent-core
• Without having PyFluent itself manage this, can set up our GitHub actions to automatically load .env files using for example https://github.com/marketplace/actions/dotenv-action or https://github.com/marketplace/actions/simple-dotenv
• Or when running tests specifically, through pytest-dotenv https://github.com/quiqua/pytest-dotenv