`create_kernel` API Update

To reduce the kwargs hell, we introduced dataclasses, which handle the settings of create_kernel and similar functions. In addition we introduced type-hints for the API functions to increase usability and simplify development.

pystencils/llvm/kernelcreation.py

+from pystencils.enums import Target
 from pystencils.llvm.llvmjit import make_python_function
 from pystencils.transformations import insert_casts
 
 
 def create_kernel(assignments, function_name="kernel", type_info=None, split_groups=(),
-                  iteration_slice=None, ghost_layers=None, target='cpu'):
+                  iteration_slice=None, ghost_layers=None, target=Target.CPU):
     """
     Creates an abstract syntax tree for a kernel function, by taking a list of update rules.
 
@@ -25,11 +26,11 @@ def create_kernel(assignments, function_name="kernel", type_info=None, split_gro
 
     :return: :class:`pystencils.ast.KernelFunction` node
     """
-    if target == 'cpu':
+    if target == Target.CPU:
         from pystencils.cpu import create_kernel
         code = create_kernel(assignments, function_name, type_info, split_groups, iteration_slice, ghost_layers)
         code._backend = 'llvm'
-    elif target == 'gpu':
+    elif target == Target.GPU:
         from pystencils.gpucuda.kernelcreation import create_cuda_kernel
         code = create_cuda_kernel(assignments,
                                   function_name,