Support for Python 3 and remote targets

This PR contains two main changes.

1. Python 3 support (which is required for point 2).

2. At some point one wants / needs to use submission tools like crab for running many fits with high parallelism. For this to work, the way that tasks handle their inputs and outputs has to be compliant with law remote targets. However, as it turns out, this is already mostly the case and there are just a few changes + 2 minor fixes required:

Tasks to adjust

• Workspace
• Snapshot
• Limits
• Likelihoods
• Significances
• GOF
• Pulls & impacts
• Postfit
• EFT
• Resonant
• Exclusion

Fixes

• The current GLOBUS_THREAD_MODEL setting makes gfal2 timeout in parallel processing (--workers >1) → Most likely another weird interference with PyROOT. Turned out to be a Python 2 issue. Fixed with Python 3.
• Remote bundles cannot be fetched after CMSSW setup in remote jobs → xrootd library in remote CMSSW env incompatible with gfal (but not in local one?) Opened #43 to follow up.

Closes #25 (closed).