Don't require a virtual environment

[YakDriver]

Is your feature request related to a problem? Please describe. Yes, it can be a pain, depending on the situation (cough, cough Azure Pipelines), to get a Python virtual environment working properly. It would be better if gravitybee could set up its own virtual environment as part of the build process rather than forcing the user to do that beforehand.

Describe the solution you'd like Run gravitybee outside of a virtual environment and it generates a working standalone.

Describe alternatives you've considered The alternative is having the user create the virtual environment, which is what happens now.

Additional context

[solubrew]

I'm having trouble understanding the virtual environment configuration for gravitybee. i'm trying to build from an Anaconda virtual environment but I keep getting the following error message gravitybee - ERROR - No virtual environment directory detected!

[YakDriver]

@solubrew There may be deeper incompatibilities between Anaconda virtual environments and virtualenv but the error that you're getting is caused by the "VIRTUAL_ENV" environment variable not being set. (virutalenv sets it when you activate a virtual environment.)

However, activating a non-Anaconda virtual environment is pretty simple (see Fix 2 in this article) and getting GravityBee to work with your virtual environment might also be simple.

$ export VIRTUAL_ENV="$HOME/anaconda/envs/<name of your env>"
$ export PATH="$VIRTUAL_ENV/bin:$PATH"
$ unset PYTHON_HOME

Then try running GravityBee. I'll open an issue for this and please let me know on issue #82 if it works or not.