Add error message that Singularity needs PIP

[wtraylor]

It says it in the README.md that Singularity is not supported by
Popper installed through install.sh, but not every user has read it.

This fixes part of #898.

[ivotron]

thanks @wtraylor, that's a great catch! One thing we can do, in order to make this more generic, is to identify this within popper itself. We can check whether we're running inside docker when --engine=singularity is given (as shown here https://stackoverflow.com/a/20012536), for example as part of the constructor of the Singularity runner. what do you think?

[wtraylor]

That’s a good idea to move the message to SingularityRunner.
I implemented it in a new commit.

I tested it like this:

docker run -it --rm -v $PWD:$PWD -w $PWD --network host --entrypoint sh python:3.7.7-alpine3.12
# Within the container:
apk add --no-cache git
apk pip install --no-cache-dir popper/src
cd /tmp
popper scaffold
popper run -e singularity -f wf.yml
# Error message comes here as expected.
popper run -f wf.yml
# No error message.

[ivotron]

thanks a lot @wtraylor ! This is awesome. And thanks a lot for taking the time to test it on your end!