Update PiecewiseLinearInterpolation functions

Integration order in project files.

Also fix the order in the initializer list.

Remove default value.

Takes around 15-20 minutes.

Supporting heterogeneously distributed DoF

[Jenkins] Run doxygen job in Ubuntu 16.04 container.

This allows for:
cmake ../ogs -DOGS_BUILD_UTILS=ON
make ctest

This will build the required executables bedore running the tests.

Fixes missing executables on clang-sanitizer Jenkins job.

16.04 has a more up-to-date doxygen package and so this
fixes some non-html-ized characters in the navigation.

SwmmInterface BugFix

Porous medium properties: permeability, storage and porosity 

Documentation