#14 Add assertions

[Peter Becker]

In some places runtime problems are caught and
exceptions are thrown were the underlying semantics is:
"someone called this in the wrong way". Although this
is nice for testing in the field, using assertions (as
in the "assert" keyword) might be better in many cases.

One nice thing to look for in a code review are thus
places were the "assert" keyword can be used in
addition or instead of the current checks that throw
exceptions.