Update documentation

I think we may want to say how to run the test and show what the expected result is to show evertthing is ok, like

```
[       OK ] DagSolidTest.surface_area_test (5 ms)
[----------] 16 tests from DagSolidTest (141 ms total)

[----------] Global test environment tear-down
[==========] 16 tests from 1 test case ran. (141 ms total)
[  PASSED  ] 16 tests
```

And then put a link to the userguide, like now we are ready to run our first problem