Gpu block size

If the Assignments act on 2D fields but 3 GPU indexing parameters are provided, an error occurs. With this MR the third parameter is fixed to 1

pystencils_tests/test_version_string.py

 import pystencils as ps
 
+
 def test_version_string():
     version = ps.__version__
     print(version)
 
-    numeric_version = [int(x, 10) for x in version.split('.')[0:2]]
+    numeric_version = [int(x, 10) for x in version.split('.')[0:1]]
     test_version = sum(x * (100 ** i) for i, x in enumerate(numeric_version))
 
-    assert test_version >= 200
\ No newline at end of file
+    assert test_version >= 1