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
@@ -520,7 +520,12 @@ class ExtensionModuleCode:
 
         headers = {'<math.h>', '<stdint.h>'}
         for ast in self._ast_nodes:
+            for field in ast.fields_accessed:
+                if isinstance(field.dtype, BasicType) and field.dtype.is_half():
+                    # Add the half precision header only if half precision numbers occur in the AST
+                    headers.add('"half_precision.h"')
             headers.update(get_headers(ast))
+
         header_list = list(headers)
         header_list.sort()
         header_list.insert(0, '"Python.h"')