Add exceptions to API

[1kastner]

Currently, exceptions are cluttered all over the code and they are not an official part of the API. If people use the API, they should be able to invoke the methods in a try/except manner and thus the exceptions should be available to them. To achieve this, first of all exceptions need to be consolidated.

The major source for exceptions are invalid input parameters. The most complex data type currently handled are distributions that are represented as dictionairies with additional constraints to be checked. Thus, this task depends on #71 and #65.