Consistent handling of optional columns

[bifbof]

The Trackintel model has two types of columns that it accesses. The required ones, without which you can't create a Trackintel object, and the optional ones mentioned in the documentation. The handling of the optional columns is rather confusing

• some like accuracy are mentioned, but nowhere used with any function
• some like trips in tours are not mentioned at all, although they are returned every time by the generate_tours function
• no information is given about what type these optional types should be
• basically no tests are written for them, e.g. for io functions

We should treat them consistently and define: which ones are really used, their datatypes etc.