Fix star general
The commit corrects some small problems with the star general stencil: - The document was not correct, as thenumber of extra points was not matching what the code does - The number of coefficient in the polynomial was too big and hence useless (193 instead of 109).
Minor bug and documentation fixes.
- I have checked that my code follows the Octopus coding standards
-  I have added tests for all the new features added in this request.