Flattened version of FDivFunExp

Revision: 
9163
Folder: 
Compiler/ModelicaFrontEnd/src/jastadd/flattening/Flattening.jrag

When optimizing a model that contains the rem() function, I got an error because the div() function (which is called from the definition of rem()) does not have a corresponding MX function declared in casadi.

It seems that I was able to solve this problem (the error disappeared) by adding the following line of code

public FExp FDivFuncExp.flatten(FQName prefix)     { return new FDivExp(new FSubExp(getX().flatten(prefix), new FModFuncExp(getX().flatten(prefix), getY().flatten(prefix))),getY().flatten(prefix));}

in Compiler/ModelicaFrontEnd/src/jastadd/flattening/Flattening.jrag

The idea is that the integer division div(x,y) equals (x- (x mod y))/y . Whether or not the implementation is complete/correct, I'm not sure! Possibly it's better to use div(x,y) = floor(x/y)? 

This is my first patch, so feel free to suggest improvements..