changed signature of getSurfaceAreaForNodes to not use pointers
added option to getMeshSurface() to keep node ids from original mesh
added function to Raster to return value at point coordinates
added general readRaster-method
fixed surface-normal requirement for surface extraction
added direct source terms with surface integration and shortened direct value method
fixed normalisation
fixed variable name + documentation for flag
transforming file extension to lower case
changes based on PR comments
Merge pull request #411 from rinkk/GuiSurfaceIntegration

GUI surface integration for source terms

Remark: The Travis CI build fails, because one test for reading and writing xml files fails in one configuration. This happens sometimes. Since this PR doesn't touch xml functionality I merge it although Travis CI is failing.