Bump version in version.txt file to 2.2.0

Forgot to bump up the version in the fallback version.txt file.
Fix bug in file-based backend of MATLAB interface

When creating a Simulator object in the MATLAB interface the version of
the underlying CADET simulator is retrieved. In case of the file-based
backend this fails with an error. In the file-based backend the version
is obtained by parsing the command line output of the cadet-cs binary.
The parsing code contained a wrong index which has been fixed.

Fixes #9.
Improve git version extraction in CMake scripts

Improves the version extraction using git tags in CMakeLists.txt. Under
Windows the version is now also obtained from git. The code has been
simplified and cleaned up.