Add experimental half precison support

With this MR experimental half-precision support is added

pystencils/cpu/cpujit.py

@@ -50,6 +50,7 @@ import platform
 import shutil
 import subprocess
 import textwrap
+import warnings
 from collections import OrderedDict
 from sysconfig import get_paths
 from tempfile import TemporaryDirectory, NamedTemporaryFile
@@ -60,7 +61,7 @@ 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
@@ -520,9 +521,13 @@ 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 = sorted(headers)
         header_list.insert(0, '"Python.h"')
         ps_headers = [os.path.join(os.path.dirname(__file__), '..', 'include', h[1:-1]) for h in header_list
                       if os.path.exists(os.path.join(os.path.dirname(__file__), '..', 'include', h[1:-1]))]