Fix ISTLMatrixBackend
This MR contains the following changes:
-
FieldMatrix<K,1,1>are now handled as a being a scalar, i.e. another[0][0]will be called if needed (see #18 (closed)) - C-style casts were replaced by
static_casts - A simple test is added (this one doesn't test more complex cases, e.g. index trees with non-uniform depth by now)
Closes #18 (closed).