make install command uninstalls user packages; guide omits the need for a virtual environment

[ghost]

Steps to reproduce

1. Clone the repository
2. Read and follow the installation guide
3. First step is make install

Expected result

The install script will install the application, without affecting the existing user installed packages or operating system.

Actual result

The install script tries to uninstall existing user packages. Because this is not permitted, this leads to an error.

pip-sync requirements/app.txt
Found existing installation: ansible 2.9.6
Can't uninstall 'ansible'. No files were found to uninstall.
Found existing installation: apt-clone 0.2.1
ERROR: Cannot uninstall 'apt-clone'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.

Possible solutions

1. Check if the installation is run in a virtual environment and stop if it is not. Even better, have the application installed in a container by building a docker image during the installation.
2. In the developer guide, a virtual environment is set up. This step is omitted in the installation guide.

[nhoening]

Thanks for the report. This is something we should support better.

I'm not sure how to check if one is in a virtual environment, but that is a good idea.

I'm also working on making FlexMeasures pip-installable, which is what most people would want to do who are not planning on developing it.
The advice to run make install would go to Develop.md.

[create-issue-branch]

Branch issue-53-Install_script_wants_to_uninstall_user_packages_guide_omits_the_need_for_a_virtual_environment created!

[nhoening]

PR #54 has landed, so the main installation is now much better (using pip) and one gets a working app. The whole trip (from zero to a useful installation) is not yet finished.

However, this ticket can be closed. I did also add a comment for developers to make sure they do use a fresh virtual environment. (see PR #72).