Extra check for displaysettings attribute

[constantinpape]

see bigdataviewer/bigdataviewer-playground#111

[constantinpape]

I have implemented a check for mandatory fields in the displaysettings attribute now:
https://github.com/constantinpape/pybdv/blob/master/pybdv/metadata.py#L7
Are there any other required fields @martinschorb @NicoKiaru ?

As a side note, I have also implemented support for passing List[int, str, bool, float] to the attributes now to support the color field. This is fine, but if we want anything more complicated, we should switch to json enconded strings as @NicoKiaru already suggested.