Be more verbose in a warning message.

In order to track down what's happening, knowing which file we were trying to write to turned out to be about as useful as knowing the processor number.
Minor edits to some text.
Merge pull request #172 from bangerth/doc-fixes

Minor edits to some text.
Create a postprocessor for complete material output.
Merge pull request #168 from bangerth/more-verbose-error

Be more verbose in a warning message.

In order to track down what's happening, knowing which file we were trying to write to turned out to be about as useful as knowing the processor number.
Better documentation.
Add a line in changes.h
Merge pull request #173 from gassmoeller/material_properties_postprocessor

Create a postprocessor for complete material output.
Function plugins for gravity and boundary temperature.
Fix small bug and improve documentation.
Add tests for function plugins.
Add line in changes.h.
Extended documentation and adjusted default value.