Field modules written in XML

All physical field information (governing equations, quantities evaluated in post-processing etc.) has been separated from the code and put in XML files. Agros2D reads all this data dynamically on demand and it has several advantages. User can easily modify equations or add quantities in post-processing without the knowledge of C++. You even compile a new Agros2D. All modules can be compiled by built-in Python compiler to avoid loss of performance.