Don't sort on groupby (e.g. in skill() method)

[jsmariegaard]

• groupby(..., sort=False) in skill() and similar methods
  • correct tests accordingly
• Fix bug in weights when assigning from dict (caused wrong results in some cases)
• Re-introduce to_dataframe() on Comparer
• Syncronize to_dataframe() of Comparer and Timeseries such that they work in the same way (include aux also)
• Docstrings for all to_dataframe() methods
• Docstrings for several other methods
• Renaming of internal variables (and variables in tests) to confirm to best practice and for consistency (e.g. sk for skill() )
• New method sort_values() on SkillTable
• xy set to nan for skill() of track data

[jsmariegaard]

Now added more tests. Okay to merge @ecomodeller ?

[ecomodeller]

Now added more tests. Okay to merge @ecomodeller ?

Good, and I changed the _to_long_dataframe a bit more.