Strip unused functions from generated code

[Martin Sjölund]

In order to speed up code generation and compilation, the backend could remove all functions that can no longer be called after symbolic manipulation.
For example, MultiBody simulations contain a lot of functions that are inlined or compile-time evaluated expressions.

[Jens Frenkel]

This should not be to complicated.

• add a new past optimisation module (copy of removeunusedparameter)

• collect all function calls in BackendDAE.DAE

• collect all function calls in collected functions

• use the collected functions for code generation 

[Martin Sjölund]

If I had time, yes :)

I'd use a work list algorithm:
Create a new Function HashTable.
1) Find all paths called in the equations, add these to the work list
2) Pop the top of the work list, or stop
3) If not already visited, add function to the new function hashtable, add all called paths to the work list
4) Goto 2

[Jens Frenkel]

implemented in 11318