Usability improvements

Look at that #124 (closed) , we seem to have a new user or at least an interested third party. Of course he did ran into issues with the very first step, the installation...

So, we already screwed up the first impression and obviously also the second. So I thought, maybe we sneak in some ergonomic improvements, to not also spoil the third. The changes proposed here:

• getResult by default now returns DataFrames for data and flags instead of a DictOfSeries and a flagger. The reasoning here is, that getResult is more a user facing method and both DictOfSeries and BaseFlagger instances are not easily digestible and probably of little use outside of saqc. There is however the possibility to opt into the old behavior.
• The validity checks for the SaQC constructor argument data are relaxed. I don't see a good reason, why we should not allow pd.Series (and simply convert them to DataFrame) nor why we enforce column labels of type string.

Comments and suggestions please @luenensc and @palmb