Combining k-values

Since we decided that optimal k-values can be different for electron and muon channels, I re-organised the file structure of the cross section results from:
```
data/8TeV/variable/xsection_measurement_results/k_value/category/xsection_channel_met_type.txt
```
to
```
data/8TeV/variable/xsection_measurement_results/channel/k_value/category/xsection_met_type.txt
```
For the combined channel k_value subfolder is omitted.
Now the k-values are provided in config/cross_section_measurement_XTeV.py as dictionaries for two channels, which can be easily updated. The 7 TeV k-values are equal to 8 TeV ones for now, until further studies.

Also:
- fixed the plotting order for main cross section plots, data always on top, then everything in the legend order.
- added the bias test regime (using MC@NLO) to k-value determination script, this yields d_i results different to the ones obtained from data.
- can make log plots in compare_unfolding_parameters.py with -l option. 
- minor improvements