Fix/#315 deviating stats

[gplssm]

Fix #315

Actually, it's not really fixed. Clearly spoken, we ship around the issue of non-deterministic results with ding0 when using Python>=3.5 by just allowing Python version >=3.6.
The problematic piece of code was not found in the end. It's quite likely that the solution is related to the following change:

Money quote from Python 3.6 what's new

[...] The order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon (this may change in the future, but it is desired to have this new dict implementation in the language for a few releases before changing the language spec to mandate order-preserving semantics for all current and future Python implementations; this also helps preserve backwards-compatibility with older versions of the language where random iteration order is still in effect, e.g. Python 3.5). [...]

Because Pandas 0.20.3 isn't available with Python >=3.7, currently, only Python 3.6 is supported.

Furthermore, it introduces

• A test that guarantees consitency of results for a real example
• A step-wise test through run_ding0 covering all testable steps
• A conftest.py for pytest that allows for example to provide package-wide fixtures
• A unique representative for LVLoadDing0
• Make all tests run again

While implementing the fix for #315, it also closes #314.

As a result and in order to clean up, I would suggest to

• Delete fix/Preprocessed data #296-cleaning-table-data