Add experimental half precison support

With this MR experimental half-precision support is added

pystencils/cpu/cpujit.py

@@ -519,8 +519,14 @@ class ExtensionModuleCode:
         self._code_string = str()
 
         headers = {'<math.h>', '<stdint.h>'}
+        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():
+                    half = True
             headers.update(get_headers(ast))
+        if half:
+            headers.update({'"half_precision.h"', })
         header_list = list(headers)
         header_list.sort()
         header_list.insert(0, '"Python.h"')