fix visualization output

compositional fields ended up being wrongly exported as 0
add test
switch test to gnuplot output and update

output changed due to velocity now being in m/year
Merge pull request #119 from tjhei/fix_data_out

Fix data out
add tests for the melting postprocessor

tjhei: switched from vtu to gnuplot output
Update years and organizations of references.
Merge pull request #121 from bangerth/fix-manual-references

Update years and organizations of references.
Minor edits to the free surface section.
Merge pull request #122 from bangerth/fix-manual

Minor edits to the free surface section.