Update to graphANNIS 0.9.0 and fix null pointer panic on repeated call to exercise

This updates to graphANNIS 0.9.0 and fixes an issue on the second time an execise was generated.

The problem was that there is a

with CorpusService.csm as cs:

block. At the end of the block, the destructor will be called on the CorpusService.csm object, releasing the internal references to the C-API and causing various problems when the invalid object used again.

Instead, a local variable cs is created as a reference to the original CorpusService.csm