Updates for new SOILWAT2 project structure

[dschlaep]

• switch to SOILWAT2 branch feature_restructure

• paths renamed in submodule SOILWAT2

• *.c -> src/

• *.h -> include

• pcg/ -> external/pcg/

• googletest/ -> external/googletest/

• makefile gained ability to create required paths at obj/ -> remove obj/ and fixed nested folder structure -> remove nested .gitignore for each subfolder of obj/ -> add obj to global .gitignore