Develop revisions

[nizomovs]

Addresses #142. The develop revisions checkout logic is now as follows:

1. if the repository has not been cloned yet:
   1. run git clone <repo_url>
   2. run git checkout <branch_name>, where branch_name is the build_version specified in the experiments.yml
2. if the repository has already been cloned:
   1. if there are local changes using (checked using git status --porcelain):
      1. if the force/confirmation flag (-f) has been set: checkout given branch by running git checkout <branch_name>
      2. if the force/confirmation flag has not been set: warn the user of local changes and refer to the force/confirmation flag
   2. if there are no local changes: checkout given branch by running git checkout <branch_name>

[c-bebop]

Looks good!