Reenable pylint

[clelange]

• fix all pylint errors and warnings that were introduced in the meantime

With a clean environment, I managed to get the tests to run on lxplus as well (using CMSSW_8_0_20 and a virtualenv).

@AndreasAlbert Can you please also try on lxplus and your setup?

Closes #43

[clelange]

Alright, so this seems to be working now. On lxplus, tests run successfully in CMSSW_9_4_6_patch1 (ROOT 6.10), but fail in CMSSW_8_0_20 (ROOT 6.06) as described in #46.
I suggest we recommend as setup CMSSW_9_4_6_patch1 and/or ROOT >= 6.10.
For things to work properly, one needs to use python -m pip instead of just pip.
For usage, no virtualenv seems necessary:

python -m pip install --user hepdata_lib

For development I'd suggest we recommend:

virtualenv HEPData -p `which python`
source HEPData/bin/activate
git clone git@github.com:clelange/hepdata_lib.git
python -m pip install -e .

If you agree, I can write some documentation.

Last question: should we run pylint on the tests as well? Currently, this would fail...

Let's merge this.