Allows codegen of generic kernal ast

Can create a kernel outside the context then generate, i.e.:

cfg = CreateKernelConfig(cpu_prepend_optimizations=[remove_empty_loops])
kernel = create_kernel(asms, config=cfg)
kernel.function_name = 'getVelocityDensity'

sfg_config = SfgConfiguration()
with SourceFileGenerator(sfg_config) as ctx:
    sfg = SfgComposer(ctx)
    VelocityDensityKernel = sfg.kernels.create_from_kernel(kernel)
    sfg.function("getVelocityDensity")(
        sfg.call(VelocityDensityKernel)
    )

src/pystencilssfg/source_components.py

@@ -121,6 +121,8 @@ class SfgKernelNamespace:
         ast = create_kernel(assignments, config=config)
         return self.add(ast)
 
+    def create_from_kernel(self, ast):
+        return self.add(ast)
 
 class SfgKernelHandle:
     def __init__(