Conda support in ZARP

Add conda support in ZARP. Users without singularity can now run ZARP using conda. Fixes #97 (closed)

Minor change: Move Snakefile from root to workflows to follow this: https://snakemake.readthedocs.io/en/stable/snakefiles/deployment.html

Since this is a new feature and I am not sure if the structure is the best. For those of you who regularly develop pipelines in snakemake (@katsanto @herrmchr @burri0000 @bakma) can you please let me know if you are satisfied with the current structure, or do you have any recommendations?

A new test is added that tests conda (tests/test_integration_workflow_with_conda/). It is similar to tests/test_integration_workflow, with the alteration between two parameters (--use-conda/--use-singularity) and the removal of paths. @kanitz Do you think that there is a way to remove this redundancy and run the test twice with the two options?