Document and test spack "dev-build" developer workflow

[ChristianTackeGSI]

Part of #261

#265 introduced a script to help with spack's dev-build command.

• We should have documentation on how the expected workflow for such a development build looks like.
• Bonus: Write a test that uses some (small) package to exactly go through all the steps and make sure they work as documented. Please ask for help here! Our test "framework" is not very well documented…

[ChristianTackeGSI]

spack 0.16 brings spack develop with multi package develop mode.
See: https://spack.readthedocs.io/en/latest/command_index.html#spack-develop
See: spack/spack#15256

Moving to "next" milestone.