revise FMI specification

[mwetter]

@tsnouidui : I merged your text to the master.

Can you please read it once again and clean up the file. For example, there is wording like
"In Listing 5.2, d2 /dt2 depends on the event indicator." -- A variable is missing, and I don't quite now what this means to say.

You need to say what index refers to in
HigherOrder index="5"
I guess that is the index of the state variable, but it is not clear. Let's be explicit to avoid misunderstanding.

We wrote now that "We will say that z=a−x is the event indicator", but there are still several locations where it says "event indicator function". We should be consistent in the terminology to make it easier for others to understand.