Add better checks on assumed input types/values to all functions

[mkruckow]

We have a lot of functions, which work currently with assumptions either only written in the doc-strings or even nowhere mentioned. All this can produce unforeseen runtime errors at any parts of the code.
A more specific example in the case of None type objects is already given in issue #236 .
Another common case are floats or integers to be of the correct type or range.