Document the symbolic transformation carried out by the various back-end modules

[casella]

The back-end modules do a lot of smart symbolic manipulations, most of which are documented somewhere in various papers. However, only developers with a long aquaintance to the code really know what they do, and we cannot expect end users to read the MetaModelica source code and perform bibliographic searches in order to find out what is done to their poor model's equations.

In the spirit of open source development, I would suggest that we write a chapter of the User's Guide where we (shortly!) explain what the modules do. As ticket:4398#comment:30 clearly demonstrates, this is sometimes not really obvious even to exper users.