Add experimental half precison support

With this MR experimental half-precision support is added

pystencils/cpu/cpujit.py

@@ -60,7 +60,7 @@ from appdirs import user_cache_dir, user_config_dir
 from pystencils import FieldType
 from pystencils.astnodes import LoopOverCoordinate
 from pystencils.backends.cbackend import generate_c, get_headers, CFunction
-from pystencils.typing import CastFunc, VectorType, VectorMemoryAccess
+from pystencils.typing import BasicType, CastFunc, VectorType, VectorMemoryAccess
 from pystencils.include import get_pystencils_include_path
 from pystencils.kernel_wrapper import KernelWrapper
 from pystencils.utils import atomic_file_write, recursive_dict_update
@@ -522,7 +522,7 @@ class ExtensionModuleCode:
         half = False
         for ast in self._ast_nodes:
             for field in ast.fields_accessed:
-                if field.dtype.base_type is None and field.dtype.is_half():
+                if isinstance(field.dtype, BasicType) and field.dtype.is_half():
                     half = True
             headers.update(get_headers(ast))
         if half: